Everything is situational. Rock > Scissors > Paper > Rock. For example, if we ignore plot no jutsu, Bee can defeat Guy, Guy can defeat Kisame and Kisame can defeat Bee. They are all lower-mid kage level. If you look at only two of these situations you can easily state that one is inferior although I believe they are all on the same level.
I dont see what Tsunade did during the pain arc besides keeping everyone alive. This feat couldnt be done by Tobirama, Hiruzen and even Minato, but they would handle things in a different way. Itachi would be able to defeat Pain. Itachi is intelligent and has a quick mind so he would have worked out the secret of the seventh pain well before Naruto ended up working it out. He would try to blind them or break their eyesight link as he did to Edo Nagato, then he would destroy the path that revives them first, then work his way through them. He could also seal them using the Totsuka Blade.
Itachi's stamina is probably the same as an elderly Hiruzen, and this is only due to Itachi's illness. He is definitely upper kage level in terms of skill, and was praised as mentally being Kage level when he was young (I think it was around 8). Itachi could solo each of the current Kage individually, and would hold his own if fighting them all at once. Just think of him as a more skilled, more experienced Sasuke, but with lower chakra levels and less stamina due to his illness.
TLDR:
Everything is situational
I am an Itachi fan, but I genuinely and unbiasedly believe he is kage level
Shinobi's official levels do not actually reflect their official levels (S-Rank Itachi and Genin Naruto and Sasuke are Upper Kage Level)