Probably around Mid-High Kage. He can beat a-lot of characters with Invis and Jinton. His main weakness is Jinton is a charge attack and Invis working against certain Dojutsu and Sensing Techs is debatable.
High kage no doubt, but where in high kage is debatable. I personally think people exaggerate how long long Jinton takes to fire. Charge time is equal to another jutsu hand seal weaving time. Also I dont think sharingan or some smelling jutsu can find Mu. Both hype and feats put him as nearly impossible to find.
Strongest he goes 50/50 with is Gengetsu, which was a mutual death in canon. Strongest he beats is probably Tsunade, due to match-up advantage. Weakest that beats him would be characters like: Hiruzen, Orochimaru, or Jiraiya.
On a tier list, he is likely top of the Mid Kage bracket, slightly behind High Kage such as Hiruzen or the Legendary Sannin.