Kisame vs Gai:

Gai wins.

Kisame had advantages such as a preexisting body of water, absorbed Bee's chakra and possible lack of KI from Gai while Gai had advantages like starting in gates, no Samehada for Kisame and Hirudora matching up well against Daikodan.

Now you can say Gai takes little time to enter gates anyway, Kisame was weakened and chakra-deprived before he stole Bee's chakra, Samehada would have made no difference, Kisame can make his own water etc and I go back and forth between which outweighs the other but narratively Kisame is the recurring personal villain for Gai that he is supposed to have defeated in the end, so I think the more correct interpretation is that once conditions are equally fair (if they weren't already the last time) Gai is >= Kisame.

You could make the argument that Samehada has set base reserves Kisame could use for healing from Hirudora, but when I look at the narrative of Samehada's healing I find that unlikely. When Kisame is shown healing from Lariat it was about using the chakra quantities he stole from Bee fitting the whole "I get stronger in proportion to my opponent/tailed beast without a tail" hype that he ascribes to himself at that exact moment of highlighting Samehada's healing. If it was a minor injury I would be more open towards the argument but Hirudora was an incapacitating injury for Kisame. Basically I'm saying that narratively it's unlikely Kisame would be able to heal from his opponent's finishing move despite being unable to steal any chakra from them.

Nagato vs Itachi:
Itachi is clearly liked by Kishimoto evidenced by portraying him as upstaging Nagato on three occasions, and having Kabuto, Nagato, Naruto and Bee all praise him and compliment him, however when we are talking clear power-relations and portrayal of strength those praises don't allude to a 1v1 between Itachi and Nagato.

So we look at the moments Itachi shined in the fight, when we take a closer look at these upstaging moments:

1. Itachi blinded the summons: this was impressive, however Itachi accomplished this while Nagato was fighting two others instead of him.

2. Itachi crushing Nagato's arm and saving Naruto and Bee: impressive but as was highlighted earlier it was thanks to Nagato's shared vision being made obtuse, which we already know was due to a distraction in and of itself.

3. Itachi figuring out CT weakness: shows Itachi's intellect, but the author had Itachi himself say they need to fire all of their strongest long range attacks at the epicenter, not one.

4. Totsuka stab: author only made it happen after having Itachi shine in three occasions specifically due to advantages of distraction and having teammates with massive firepower.

The most praise we can give Itachi is that the two Jins would have lost without him, but the author made it painfully clear Itachi alone wouldn't have sufficed when every single one of his good showings against Nagato was due to an advantage. In addition the author had Kabuto complain about Nagato's mobility, made controlled Nagato neg the surprise Amaterasu (that he sensed earlier) and (iirc) said something about forgetting about Itachi.

Itachi can be argued to have been holding back against literally everyone he's ever fought on panel; PI Jonin (Kakashi statement that he's not showing his true power, Kakashi implying Tsukiyomi could have been lethal, DB implying somewhat Itachi wasn't going all out), Shoten vs Team Kakashi (Itachi smiling at the end), Hebi Sasuke (planned to lose), Kabuto (no KI because he needed to stop Edo Tensei) - except his fight with Nagato...

I don't think the author's choices of how the battle between Nagato and Team Itachi was carried out shows Itachi as above Nagato's level, I think it explicitly shows the exact opposite.

Kisame vs Hachibi:
Kisame could win if he manages to fuse and latch onto the Hachibi, however the waterdome may not be a good option as Hachibi would think to nuke it. Maybe fusing in a lake and flanking the tailed beast might work.

Kisame is a bad match-up due to his absorption, but A3 who's an entire tier (and a bit) above Kisame stalemated Hachibi so Hachibi has decently superior portrayal.

There's also Kisame being > V2 Bee, and although wild Hachibi might be > V2 Bee there's the perfect Jinchūriki factor that amplifies the beasts to narrow that gap. But then again, Kisame's match-up advantage over V2 Bee is more severe than his advantage over the Hachibi due to fighting style (CQC whereas Hachibi has more options and range).

Hachibi likely wins I think, Kisame could win if he didn't mess around and hachibi had zero knowledge.

Itachi vs Jiraiya:
This is not everything I think on this match-up and moreso a long summary - maybe sometime later I will edit it and add the details. For now this is a rough draft.

There's no direct evidence or statement that Itachi's statement about Jiraiya was a lie, just one people extrapolate from Itachi's reveal as a good guy. More importantly there is seemingly contradicting portrayal in that Itachi is > Orochimaru.

You see you can also argue Jiraiya > Orochimaru based on portrayal but it just seems like Jiraiya has been consistently portrayed as "on the same level" as Orochimaru, even if slightly superior. Kind of like Naruto and Sasuke.

1. the DB says Jiraiya is "on par" with Orochimaru in strength, "on par" is not the word I would describe Itachi and Orochimaru's power relationship.

2. Itachi is able to tell Orochimaru that "all his jutsu are useless before him", Jiraiya never had such portrayal to factually talk down to Orochimaru like that.

Basically the biggest issue in Jiraiya vs Itachi is Orochimaru, leading to Itachi-fans hyping Orochimaru in relation to Jiraiya (Orochimaru is > or = Jiraiya) and overrating the gap between Itachi and Orochimaru ("Itachi one-paneled Orochimaru twice as a kid!") Wh
ereas the Jiraiya-fans de-hype Orochimaru in relation to Jiraiya (i.e claim Jiraiya >>> Orochimaru) and underrate the gap between Itachi and Orochimaru ("Itachi needed Susano'o and Totsuka to seal Orochimaru so the gap must be small!")

Jiraiya has paralells with Itachi but he also has parallels with Orochimaru.

For me personally Itachi's superiority over Orochimaru has more statements and feats supporting it, not saying Jiraiya and Itachi's equality has no evidence for it just that Itachi's superiority over Orochimaru is emphasized more and therefore when two seemingly conflicting evidences co-exist I'd favor the one emphaized and repeated more.

The gap between Itachi and Orochimaru while not big, you can definitely make an easier case for the gap between Itachi/Oro to be bigger than Jiraiya/Oro. Just imagine if Jiraiya could tell Oro "all your jutsu are useless before me" or Oro emerging and then getting beat by SM Jiraiya swiftly who then smiles and says "is that all?" - most people would think the gap between the two Sannin is huge.

However, the statement still exists so it's a matter of abundance of evidence rather than lack of evidence on Jiraiya's part. I'd say Sick Itachi is at worst Jiraiya's equal and averagely superior to Orochimaru, and at best averagely superior to Jiraiya and decently superior to Orochimaru.

So basically I respect both those who believe Jiraiya = Sick Itachi and those who have Itachi > Jiraiya, but I think both sides need to be understanding that there are good arguments for both.

Then we get into Edo Itachi.

Edo are not brought back stronger than they were, in fact the opposite is implied. Yes Itachi didn't have his sickness but it's not like he was born with it, in fact the fact that Itachi is brough back without it means there was a time he wasn't sick while alive.
The point I'm trying to make is that Itachi at some point in his life could replicate everything his Edo self did (except Edo regen). Not only that, but Zetsu outright says on-panel that Itachi should have been "much stronger" than we've seen. The author would have also highlighted that Edo Itachi was at a level Itachi never achieved before, yet there was zero implication he was this reborn MS-spamming beast - and we're
just left with previous ET regen hype as an advantage.

People will say "but Itachi probably didn't master MS back when he was healthy since he became sick while still learning to master it" but that doesn't matter since in the end Itachi at 13-15 years old was still portrayed as outright superior to Orochimaru meaning they're indirectly suggesting Itachi with no MS experience still embarrassed Orochimaru.
In fact the whole implication from "we don't know if Itachi mastered MS back then" is to suggest that Healthy Itachi might not be much stronger than Sick Itachi or not stronger at all since he might have traded health for MS experience, the problem is the manga directly shuts this down by stating Itachi should have been "much stronger" than his sick self.

So healthy Itachi does exist and he does have feats in the form of Edo Itachi and just because not every feat is 100% transferable doesn't mean we get to ignore Itachi was once healthy and stronger than what we've seen.

In conclusion; Healthy Itachi is at worst averagely superior to Jiraiya and decently superior to Orochimaru and at best decently superior to Jiraiya and massively superior to Orochimaru.

Unpopular opinions:

Deidara is a level below Hebi Sasuke
Jiraiya can't beat the three paths without Frog Song

Kakashi is much weaker than Itachi. 3T Itachi would mid-diff 3T Kakashi.

3T Itachi is on the same level as the mid-tier Akatsuki generally speaking, whereas Base Jiraiya is a level above them.

Shoten Itachi was jobbing against Naruto & co

All of Hebi Sasuke's feats/portrayal are by far the most undermined in the NBD.

Gai needs the 7th gate to beat Kisame.

Kisame is weaker than Base Jiraiya.

A Doton cannot overcome Daikodan.

A boss summon is decently smaller than the waterdome.

Kisame has a chance to beat A3 who's a tier above him due to match-up, if A3 is assumed to be like V2 Bee (but it's not concrete).

Jiraiya needs frog song and a favourable location to beat the Preta, Animal and Human Paths.

Jiraiya can step up to Muu and can beat him.

Jiraiya having Katas and sensing doesn't make him stronger than what he was already portrayed.

Jiraiya is stronger than Prime Hanzo.

Jiraiya is stronger than A3.

Base Jiraiya would beat WA Kakashi.

Gama the toad is somewhere in the low-Jonin level.

Katsuya's combat strength is at best on Manda and Gamabunta's level and may be weaker than them.

Ribcage-Susano'o Sasuke is easily stronger than Raikage 1v1 in a fair fight.

Gengetsu vs Mū:
They probably started in CQC, Gengetsu with water gun and Muu with his blades. Gengetsu manages to get the upper hand and set up a water pistol, only for Muu to sense it and evade last minute. Taken aback by this, Gengetsu leaves himself open for a sword slash by Muu, which goes through his water body leaving him undamaged.

Both Kage fall back praising each other's defenses. Muu then taunts Gengetsu as he goes invisible, to which Gengetsu responds by summoning the clam, taunting back that Muu isn't the only one who can hide.

Muu, who's invisible, notices the appearance of several Gengetsu bodies on the battlefield. He contemplates whether they're bunshins or an illusion, but since he can't sense any disturbance in his chakra he presumes it's the former and proceeds to re-appear severa
l feet in the air and charge Jinton which he concluded was the perfect way to deal with their numbers. Gengetsu realizes that Muu can't use Jinton while invisible. As the Kekkei Totai beam obliterates the battlefield and the dust clears, Muu sees the doubles are still there, much to his chagrin, before narrowly dodging another water pistol ambush.

Muu starts to rethink things, whether the doubles are real or not, whether it's an illusion or a powerful ninjutsu that Gengetsu possesses. He decides to use Jinton again, and pay more attention to the doubles, as close range combat would leave him more open than being on the air.

After a bit, the battle reaches a stalemate, where neither seems to find the other. Muu concludes that whether this is an illusion or a ninjutsu, as his sensing powers can't break him out, the only way must be to find and destroy the clam. The invisible Muu goes to the ground, and walks between the doubles unhindered. He decides to re-appear and charge a big Jinton beam to destroy the ground, instead of the doubles, and hits the mark. The doubles disappear, and Muu attempts to sense Gengetsu's whereabouts, and he does so successfully. Although the Jinton and invisibility took a chunk of his chakra.

Gengetsu, hiding behind a distant rock pillar, congratulates Muu on figuring out his clam jutsu, and tells him he's got more under his sleeve. Gengetsu then summons Joki Boi. The imp quickly launches himself at Muu, who decides to not risk a Jinton to conserve chakra, and attempts to go invisible, barely making it in time due to Joki Boi's unexpected speed.

As an invisible Muu closes in on the imp, he senses an usual buildup of chakra inside the imp's now bigger body, and barely manages to dodge the range of its explosion with minor injuries.

The battle continues with Muu withholding his Jinton for a perfect moment, finally managing to outsmart the imp by throwing his blade and distracting the imp (similar to base Bee doing something similar to Kisame in a pincer attack with his thrown blades), reappearing outside of the imp's line of sight, and charging a huge Jinton to cover for the imp attempting to dodge as Muu has witnessed first hand how fast the imp was. The Jinton manages to tag the imp, destroying it.

Muu's chakra with that last Jinton was below half, so he couldn't use Jinton anymore, and was running low on reserves and getting fatigued. Gengetsu, despite barely engaging directly, is also drained from the passive usage of his clam and Joki Boi summons.

They engage in a last ditch effort CQC scuffle that, despite Gengetsu having a slight skill advantage, Muu being able to go invisible gains the upper hand. Although to no avail as Muu has no way of winning unless he can outlast Gengetsu's water body.

Eventually both Kage succumb to their death in CQC after running out of chakra, ending a glorious battle.

Kakashi vs Orochimaru:
Orochimaru wins, probably low difficulty.

Kakashi can't do much to hurt Orochimaru as his offense is easily handled by Orochimaru's passive defenses ala WSP and Kamui will be needed to even survive a lot of Orochimaru's arsenal. Manda and Orochimaru attacking Kakashi together will overwhelm him pretty quickly and Yamata isn't needed.

Without Kamui, Kakashi was so far below Orochimaru he didn't even dare face him, and Orochimaru literally mocked Kabuto for being only as powerful as Kakashi. Kakashi improved his non-Kamui arsenal slightly throughout PII, but nothing to really make a difference as we saw he was praised for mastering his MS instead rather than his jutsu or physicals. So Orochimaru no-low diffs 3T Kakashi and low-mid diffs unrestricted Kakashi.

Kisame vs Mei:
Kisame already defeated a Yoton user, and Mei's suitons can be countered by Kisame's suitons which are bigger and can be spammed more. Hidden mist isn't an issue for Kisame because he is a swordsmen of the mist (Kakashi stated Zabuza is making the other swordsmen "dangerous" in the mist, rather than restricting them or holding them back) and because Mei isn't a master at silent killing. The only reason she might opt for the mist is so she can cast acid mist in conjunction with it, but not only is the acid slow to take effect, Kisame can just dive underwater in his lake to avoid the effect and have his 1000 sharks swarm her (Asa Kujaku
stalemated a bijuu's attack in aoe but failed to match 1000 shark's aoe). Worst for Mei she has no reliable answer for Daikodan and/or Waterdome.

Kisame losing in CQC to acid mist is unlikely for several reasons. It is not guaranteed that he wouldn't start with ninjutsu. He started with ninjutsu against team Gai and 6G Gai on turtle island, only starting with CQC against Bee and Asuma so it's a 50/50. Even then in both cases we could argue that either he was told not to use big jutsu by Itachi or that he was maximizing Samehada's effect on the Jinchuriki cloak. Furthermore Kisame doesn't have to start the battle offensively with suitons, summoning his lake and then engaging in CQC is still relatively safe for him as he can go underwater. This is assuming Mei will go for acid mist when he's charging her and that Kisame will not retaliate by using ninjutsu (Kisame used ninjutsu twice in the midst of CQC against Gai and Asuma, and his clone interrupted Neji and Lee's ambush with ninjutsu, yes they are not strong but they are fast and skilled in CQC). Kisame also should have knowledge on Mei's abilities, probably.

Based on portrayal, Mei doesn't have anything that definitively puts her above Kisame. She is a Kage, but Kisame was matched closely with Gai as his long-time rival and needing his trump card to be finished off by him; Gai is relative to Kakashi who an arc earlier was confirmed as Kage-level when he was considered for the hat. So Kisame is also Kage-level. Furthermore, she was portrayed as performing the worst against her Susano'o clones, a liability to Gaara, outperformed by Tsunade who has the title of the strongest kunoichi instead of Mei; so Kisame being weaker than Gaara/Onoki/Tsunade/A4 doesn't mean much, because Mei also is.

Kisame wins.

Kisame vs A3:
Like I said first we need to look at their portrayal to determine a winner, A3 stalemated the Hachibi and gave numerous shinobi of the alliance trouble despite them having knowledge on him, Kisame would never give that much trouble to war arc Naruto clone or not. Kisame on the other hand already admitted inferiority to Jiraiya who is weaker than SM Naruto and his rival is someone said to get in SM Naruto's way against Pain (even if you disagree with that SM Naruto is still generally portrayed as above Gai). SM Naruto would never be forced to use danger sensing to beat Kisame. So from a portrayal aspect A3 > Kisame.

However when it comes to Kisame we have a direct on-panel fight clearly showing us Kisame can be a bad match-up to certain characters that are above him in strength like Killer Bee (yes Killer Bee even without BM is still superior to Kisame because he is more dangerous and can beat more shinobi than Kisame can).

Now the question is what is the profile of the shinobi that Kisame is the ultimate hard-counter for?
1. wears a chakra shroud
2. fights mainly in CQC
3. has a ton of chakra to supply Kisame for healing and to empower himself (ironically both A3 and Bee have biju-level chakra)

A3 fits this nicely, you could argue V2 Bee and A3 are very similar in how they fight and thus Kisame would be able to replicate his performance against Bee.

So one could argue Kisame keeps soaking the damage and getting stronger while A3 gets weaker, and we know Kisame almost drained Bee who has the reserves of the entire Hachibi clean so A3 won't take much longer. A3 also is very straightforward and doesn't have many options. Kisame has also absorbed Raiton in canon too so the lightning armor isn't a problem.

However there are still problems with this view, one is that the portrayal of Nukite's strength could be argued to be above V2 Bee's Lariat and that Kisame wouldn't survive it, another is that we're still only assuming that Bee and A3 are similar enough that it wouldn't make a difference against Kisame while in reality any difference in CQC skill, reflexes, and more importantly the strength of Nukite could make enough of a difference that Kisame doesn't actually win. Another problem is that A3 supposedly has "Black Lightning" that we have to account for...

Overall, I think the more logical and fair conclusion would be to say A3 can potentially be in trouble against Kisame due to how he fights, but is more likely to come out on top due to his superior portrayal, unseen black lightning and any minor advantages he might have
over Bee in CQC skill/speed/reflexes/physicality.

Hebi Sasuke vs Kisame:
Sasuke wins.

In CQC Sasuke is faster, has sharingan, CS and survivability. Kisame's strength likely won't be a bigger factor than Sasuke's CQC snake ninjutsu, 3T genjutsu and CS amp.

Kisame's Suitons can be dodged or blocked by snake meatshields, giant snake summons or Manda. Kisame will be hard pressed to deal with both Manda and Sasuke which could leave him very open to subtle genjutsu (like Deidara), sudden CS amp, and snakes. If Sasuke is severely wounded he can use Oral Rebirth, he could use it even as a surprise attack like he did against Itachi. Kisame could outlast but Sasuke can use Kirin with no chakra left, so even that route is not guaranteed for Kisame.

Kirin, Manda, 3T genjutsu and CS2 give Sasuke the win. Basically Sasuke can match him directly whereas Kisame will be hard pressed against Sasuke's indirect tricks (genjutsu, CS, Oral Rebirth).

Sasuke is generally stronger as he has a Kage-level boss summon, an S-rank finisher that while requires some prep can be used with no chakra left, a Senjutsu boost and the powers of the white snake. Mid-level Akatsuki like Deidara are below a restricted Hebi Sasuke and restricted high kages like Sick Itachi can be pushed by him.

Kisame vs Deidara:
Kisame is much better in CQC but he isn't fast enough to completely prevent Deidara from reaching the air. Once in the air Deidara will pressure Kisame with C1/C2, Kisame can dive underwater to escape the explosions but Deidara can create Clay Fish which is big trouble for Kisame.

Kisame's offense will be shark missiles in order to reach Deidara but those can be dodged by his bird while Deidara counters with bombs. Daikodan could have enough aoe that Deidara can't dodge it but it isn't so fast that he can't match it with a C2 bomb that the shark can't absorb.

Deidara probably high-diffs, Kisame not being able to absorb the Clay screws him here. Although there is a chance Kisame could injure Deidara on the ground seeing as the distance is only 5 meters.

3T Itachi vs Kisame:
Could go either way, with full knowledge Kisame will try to avoid CQC and focus on long range ninjutsu while Itachi will try to do the opposite.

Really the match depends on who succeeds in pressuring the other into their range, which is a toss up. Kisame would destroy 3T Itachi with long range ninjutsu while Itachi is skilled and fast enough to finesse Kisame with his powerful 3T genjutsu in CQC that Kisame likely won't be able to break in time.

In terms of portrayal 3T Itachi shouldn't be underestimated; he could startle Orochimaru with his power (compare that to Orochimaru disrespecting PI Kakashi's power) and he was overwhelming Kakashi in PI (who was said to not be a low-diff for Kisame). Additionally Itachi's clone without MS beat IA Naruto who was said to have surpassed IA Kakashi.

Orochimaru vs Kisame:
There are three stages to Kisame's fights (although the first one might be skipped straight to the second like his Shoten did); first is on the ground (kinjutsu and occasional Suiton if water is available like against Asuma) on the lake/body of water (kinjutsu and more frequent Suiton like against team Gai and 7G Gai) and inside the waterdome (Bee).

If Kisame engages Orochimaru on the ground he will be overwhelmed quickly, Summons are a problem without ninjutsu, Orochimaru's sneak attacks are a problem (blindsiding KN4 post-TBB), bindingsnake glare is a problem (Hebi Sasuke subdued both partial-transformation Jugo and Suigetsu with it, while Orochimaru restrained a massive branch of the God Tree). On the lake he stands in a better position as Orochimaru's underground tactics and snakes will be tougher to land and he can dive underwater and try to dictate the battle from there.
Kisame is going to be constantly pressured from match start by snakes, Kusanagi, and poison (that he likely can't heal) while on land. If he finds room (likely unless he has no knowledge) to use his lake (or start with it like we've seen him do against team Gai) then he gets a breather. From there Orochimaru amps up the tempo with large summons that could likely swim (Manda 2 could swim) and Kisame gets overwhelmed; 1000 sharks is matched somewhat by 1000 snakes and Daikodan is blocked/meatshielded by Manda/Hydra/Rashomon, the rest of Orochimaru's jutsu (Kusanagi, Oral Rebirth, giant snakes, poison) trump Kisame's (shark bullet, water prison, feeding sharks (few in numbers) meaning Orochimaru is putting the pressure on Kisame on his own lake.

Kisame will likely die before he gets off Daikodan or waterdome, I would say his chances of surviving until he gets them are about as equal as his chances of being killed off neg-diff in initial CQC exchange I.e unlikely. If he did manage to get them off Orochimaru can probably Oral Rebirth out of being grappled and drained, from being ripped by the sharks and right before he drowns to get a new pair of lungs. Orochimaru can probably still fight from a distance using Manda or other snakes, and if Yamata comes out there's not much Kisame can do to something bigger and stronger than Manda has eight separate giant heads, regen (presumably) and has Orochimaru himself inside of it to also fight.

Overall a low/mid diff fight for Orochimaru, if Orochimaru used more jutsu that were absorbable, or wasn't as indirect in his fighting style Kisame would have stood a better chance. Orochimaru is also smarter, more knowledgeable, more experienced, has more unknown jutsu hype, knows more elements, has a perfect score in genjutsu, etc. Plus Kisame said Jiraiya was stronger than him, with the wording indicating the reason is because he's a Sannin, and was said to not be able to defeat PI Kakashi scot-free (something like mid-diff-ish, you can argue it's higher but definitely not a low diff) whereas Orochimaru was so far above PI Kakashi that they were almost incomparable as indicated by Kakashi's reluctance to even face him (neg/low-diff)

Jiraiya vs Kisame:
Jiraiya is primarily a mid-long range fighter who relies on those ranges to deal damage and locations to set traps or activate jutsu and has large aoe techniques in combination with his summons that he can afford to use because of his stamina, he has SM on top which enhances all of the previously mentioned traits and the added bonus of also having short range as a much more steady option as well as a one-shot genjutsu.

Kisame is mainly a CQC fighter that can still operate in mid-range by using his suiton and long-range with his bigger guns/trump cards (Daikodan/1000 feeding sharks), his stamina coupled with his absorption and healing allow him to fight a prolonged battle however the absorption and healing are less reliable than his stamina since they depend on the opponent's fighting style to occur. He has no special defense, no extraordinary intelligence/knowledge and his basic ninjutsu is not versatile and he mainly depends on an anti-ninjutsu blast in the form of Daikodan and sometimes he fuses with his sword (most likely to capture but might use it to kill if need be?).

When we look at their portrayal, we know Kisame admitted inferiority and was almost killed by Toad Stomach, yes Jiraiya was advantaged because of the building for that specific jutsu but if Kisame was equal to or superior to Jiraiya at full power he wouldn't have struggled to survive even with that disadvantage against Jiraiya so easily. It’s a location advantage not a sharingan power up or something; it’s not like Jiraiya is a tier stronger when he’s in a building.

Kisame usually likes to engage in short range cqc first, or at best we can assume he'll use mid-range suitons after he supplies himself with the lake like against team Gai. Jiraiya on the other hand uses mid-range combat abilities. If Kisame decides to engage Jiraiya on land, he will either get killed or severely injured by Jiraiya's jutsu. Jiraiya's oil, Yomi Numa, Wild Lion's Mane etc are all a problem for Kisame who has nothing to rely on but his strength and cqc. Assuming Kisame starts with the lake he makes things easier for himself, but the problem is even Jiraiya's small toads have feats of sneaking up on shared vision (when Jiraiya got out of that small toad's mouth). Jiraiya's boss summons have large aoe that Kisame can't counter other than Daikodan and they can likely swim efficiently too, the
toads can fight in CQC with their weapons or clear distances by jumping high or side-stepping like against Shukaku, a Biju-sized Gamayu Endan gets no answer from Kisame other than a well-timed Daikodan. 1000 sharks can be cleared by the toads aoe plus Jiraiya's attacks. Kisame is stronger than Gamabunta but he needs to focus on him to actually beat him and he’s still got Jiraiya to worry about.

The above is basically saying that Kisame is absolutely struggling against Base Jiraiya and one of his boss summons and needs his trump card to even survive realistically, this means that there is a chance the fight will end here before progressing into the “next stage” for Kisame (waterdome and fusion) by a blindside from Jiraiya; just like there was a chance before that Kisame would be defeated or injured on land before he decides to use his suitons.

This is an important point here because characters don’t start out with their stronger jutsu and instead pick up the pace, so there’s a chance that they will get defeated or injured before the fight escalates depending on the gap between the two characters or more specifically how strong their "casual" arsenal is.

Examples from the manga are when Orochimaru came out of Sasuke and used his Hydra, he was ready to engage Itachi with his Kusanagi (and presumably other snake powers he didn’t need his arms for) however he was stopped short before even engaging Itachi and sealed. If we’re being optimistic towards Itachi this implies a decent gap between him and Orochimaru in power, and if we’re being pessimistic this implies there’s a decent gap between their trump cards (Susano’o, Yamato), but the point is that there is a gap of strength that made one character die before he tried everything he had. Better yet look at Jiraiya vs Konan.

An example of injury would Asuma getting injured by Kisame before he really used Hien to its maximum effect nor used his Katons, and Asuma was going to be killed/incapacitated/injured further from Kisame’s water shark missile if not for Kakashi – Asuma’s cqc skill is high and he’s been praised for it so he could scratch Kisame with Hien but his other abilities pale in comparison and so does his portrayal which is why he was cast aside (or almost was) by Samehada shaving and shark missile respectively before he showed us everything he had.

Moving on, it's apparent that Kisame is outmatched and outgunned even in his lake both in direct combat and indirect combat (blindsides, numbers, distractions, traps) and all he has left is Samehada fusion which might help with surviving a few attacks but realistically Kisame can still be blindsided, dragged into an acid barrier, falls for a toad distraction, etc. The problem for him is Jiraiya’s attacks aren’t naturally easy to absorb and he’s still vulnerable even when fused, if he latches on to Jiraiya the latter might draw in NE and turn him to stone. The biggest problem is that Kisame might very well die before he fuses or more accurately before he decides to fuse; waterdome combo is more suited for shinobi like Killer Bee and Daikodan/1000 sharks are a more direct threat to shinobi than chasing and draining them.

There’s no need to get into SM Jiraiya vs Kisame because Kisame can’t realistically stop Jiraiya from reaching it if he wanted to since he isn’t nearly as versatile, smart or has what it takes to go through Jiraiya’s traps, and he can’t really last long against him. The feints SM Jiraiya against the 3 Pains in CQC would obliterate Kisame and so would COR and most of his attacks, worst of all Kisame can’t absorb any chakra since he’ll get turned to stone and Jiraiya being the more knowledgeable experienced shinobi might use that on Kisame.

To conclude, Jiraiya needing Gamabunta and a few other jutsu might make this a mid-diff fight for him, but the fact that he is still not fighting with his full moveset and ignoring the power up SM gives him entirely I’m inclined to say it goes back down to low diff. This fits with the portrayal of these characters and takes into account their movesets.

To summarise the above: Jiraiya wins low difficulty against Kisame because:
1. Kisame could die or carry an injury before his big guns are out.
2. Jiraiya doesn't need SM to win.
3. Kisame needs his big guns to survive against Jiraiya's normal arsenal.
4. Kisame can't pressure Jiraiya enough to raise the difficulty.
5. Kisame can't reliably absorb/heal due to how Jiraiya fights.
Jiraiya vs Masters:
Jiraiya (unrestricted Jiraiya starting in base) is a solid high kage for me, SM Jiraiya (unrestricted Jiraiya starting in SM) is an above-average/strong high kage and Base Jiraiya is an average mid-kage. Either master is an average low kage.

A high-kage on average should have no trouble low-diffing a low-Kage, so two of them being defeated with high difficulty isn't a stretch. The issue here is the teamwork Gai and Kakashi possess and how much difference it will make and even more importantly that Jiraiya momentarily becomes a weak mid-kage or even arguably a strong low-Kage while prepping for SM so depending on how the fight goes and how aggressive the masters are Jiraiya might carry an injury or lose a limb as the cost of reaching SM.

Overall I give the edge to Jiraiya with extreme difficulty as a high-kage, once he reaches SM if he has a boss summon out (he should) the masters are walled unless Gai can pressure Jiraiya enough for a successful Kamui which I doubt given Jiraiya's numbers (clones/summons occupy one master, Jiraiyaoccupoes the other), superior stamina (infinite Senjutsu vs taxing gates) and ability to keep Gai in long-range (Gamayu Endan, Water Gunshots, Leaping huge distances).

Oro w/ET vs MS Sasuke:
I think Orochimaru is somewhere between MS Sasuke (strong high kage) and Hebi Sasuke (average mid-kage), Orochimaru is relative to another high-kage in SM Jiraiya (average high-kage), albeit slightly weaker - making Orochimaru a low high-kage.

MS Sasuke on average would defeat Orochimaru with somewhere between mid and high difficulty , it's not low because they're in the same tier, and it could be high because of match-up (Amaterasu resistance, etc).

Orochimaru then adds two Hokage levels to his power, the "Hokage" distinction implies they're not at the lowest levels of kage; so at worst they are not Entry-Kage level comparable to Chiyo, IA Kakashi/Naruto, but rather low-Kage comparable to Kisame, Mei and the Masters - at best they are mid-kage or higher.

Sasuke wil need to bulldoze through Orochimaru's summons and defenses in order to win, while simultaneously watching out for Mokuton, Suiton, Bringer of darkness that can trap kages and seems to not be broken even by partner method, among their other jutsu.

Another issue is actually putting all three Kage-levels down, Amaterasu seems to be a counter for Edo regen, however Hashirama could likely push the flames with Mokuton and Orochimaru Oral Rebirths out as per canon. Genjutsu is not much of a threat here because of the presence of three Kage-levels who can use partner method which is a canonical good counter against 3T genjutsu, we also saw Onoki bring a Kage back into the fight after Madara's genjutsu seemingly incapacitated him. Tsukiyomi would have been great here if Sasuke had it as it has the power to mind-control high-kages which would be a decent counter to Edo Tensei.

With the Edos not toying around like they were with Hiruzen, they probably could use more jutsu and with Orochimaru joining the fight Sasuke is gonna be hard pressed to find an opening for a sealing tag on the Edos, and an opening for Kirin for Orochimaru.

Orochimaru wins mid-diff, high diff at worst, eventually Sasuke runs out of chakra due to needing Susano'o so much to survive, or they find a way around his Susano'o while pressuring him/distracting him. These two outcomes are much more realistic than Sasuke soloing a high-kage + two low-kage minimum where his best jutsus are unlikely to overcome the numbers and counters they have and is needed for defense to not be overwhelmed.