Might Dai and probably some random dude. Definitely not Itachi since he's no where near as good at taijutsu than Gai or the people Kisame is referring to.
I hope he wasn't referring to Itachi, since that'd be a major insult to Gai! Lol.
But seriously, if I recall correctly, Kisame is slightly younger than Gai, so the chances of him fighting Might Dai. It's probably something that'll never get explained.
He didn't really know Might Dai, and Might Dai didn't even kill every Swordsman.
One is probably Itachi even though he's nowhere near as good in Taijutsu as Gai. He's still graceful and very good though, and Kisane did have great respect for him.