Worstgen
Shiroyru
Itachi
Shiroyru
He won’t beat Itachi 100% of the time, but he has a real shot at winning since he has counters to most of Itachi’s hax. Amaterasu? Samehada can absorb it. Susano’o and Yata Mirror? Just create a mountain sized dome of water, they won’t protect Itachi from drowning. Any genjutsu besides Tsukuyomi? Samehada breaks him out. Fire style jutsu? Kisame has the type advantage.
Shiroyru
The only thing he really can’t counter is Tsukuyomi, but Kisame knows better than to look Itachi in the eyes. Even if he does get caught, Kisame should have the mental fortitude to keep fighting, given that he didn’t even react to having his sharks eat him alive