Worstgen
TheAncientGorosei
Depends on the version of Itachi. Kishimoto was very clear with the fact that an uninhibited Itachi > Pein. Had Zetsu, who just witnessed Pein kill a Sannin, call Itachi "invincible".
Finalbeta
Nagato deletes him
Sentinel
Imo all 6 Pains win against Itachi but I'm sure Itachi could at least defeat Tendo Pain.
Bogard
Pain was a tier stronger than Itachi. Animal path with the summon spam, while hiding in the invisible one alone will be tough for Itachi to handle without MS. Just for cerberus, he'd need amaterasu, and for the rest, he'd need Susanoo. In the long run, Animal Path's summon spam game can outlast him.
Bogard
Now you add Asura Path's destructive power, Preta Path to absorb ninjutsu, all that with the shared vision, and it already becomes problematic especially with the shared vision. If Tendo comes in play with his huge shinra tensei or chibaku tensei, it's game over, and he still has tricky jutsus to steal soul or revive people