Kisame is the most powerful by a laaaaaaaaaarge margin. Just look what he can do with Suiton, he is pretty much the only swordsmen that can play with the big guys.
I don't think the others are that strong. Zabuza is nothing more than a starter villain, today his skills are far outclassed by most characters and the only one of his skills still worth noting is his silent killing. The other swordsmen are even less impressive if you take into consideration their performance in the manga, they were dependant of Zabuza's mist to slay people down and as soon as it was gone they were off-paneled one after another.
Of course, if you are taking into consideration their anime skills, then Ringo becomes the second strongest for being the only one to not use his edo regeneration in his anime battle and Zabuza scores the third place with ease. I'm still waiting to see Mangetsu in action, I think he may be able to steal the second place for Ringo, but I will wait to see what the anime does with him.
So far I'm not definitely not impressed by Jinin (in my opinion the weakest) and Fuguki Suikazan.
I think it is beatwean Zabuza, Aymeri, and Kisame 
