I know Madara is stronger, but he just hasn't been hyped quite as much as Itachi. Itachi has this...feel about him. This force of prescence that makes one want to slap all the 'tards who think Jiraiya stands two shits of a chance against him. We've seen him on the losing end against Sasuke several times, but only due to very specific circumstances, which were obviously tacked on by Kishimoto so that Sasuke would actually put up a close fight. Even now, Sasuke's left exhausted and Orochimaru came out, attacking as a separate entity, thus making the match no longer one-on-one. So even if Itachi still ends up losing, he at least has the honor of having beaten a Sannin and nearly beating someone who was also around that Sannin's level in one fight.
Itachi is just not the kind of person that any one ninja can beat. Even Sasuke, with his grandiose PnJ, is currently helpless. Oro lasted three pages, yet again. Itachi is just so much hax, it borders on the impossible.
When reading the topic title...I thought you meant which character physically looks the strongest(even if they aren't the strongest, just which one looks that way). And in that case I'd say Kisame because he's a big guy with a lot of muscles and a huge sword that a lot of people probably couldn't even lift.
Anyway though, after seeing more what the post was about...I'd say Pein. lol.
As much as I hate him, probably Sasuke. He's been in several fights so far in Shippuuden, and not once has he taken a direct hit (at least not to my recollection). Every injury so far, he's either caused himself (chidori in the Deidara fight), was a result from exhaustion, or received while dodging most of an attack. Even against Itachi he's been largely untouchable.
Pain seems like he is stronger than Madara since it was said he is undefeated and his fight against Jiraiya was impressive. Also the fact that we know Madara has already lost to the first hokage along with getting his plans spoiled by Minato later.