So we've seen Kisame, Zabuza and Raiga; three members of the 7 swordsmen. All had a partner. Kisame's in the akatsuki who are very powerful ninjas in their own right. Kisame also seems to have an enormous amount of chakra, possibly kage level amount. Itachi is also his partner who...well yea you all know about him lol.
Raiga went down pretty easily and didn't show as much potential with his jutsu's. Although he might win against Kisame since he was electric based but Kisame might have more skill. Raigai also had that child with the freaky powers.
Zabuza was just a bad ass character to begin with using assassin like jutsus and techniques. Also had a better partner who could hold his own but he had a big weakness which lead him to lose.
Is kisame the strongest? he's still alive after all.