1 - Mei
2 - Tobirama
3 - Kisame
I honestly think Kisame is over-rated. Yes, he can use large scale techniques in areas with no water but I feel thats more to do with his large chakra reserves than skill.
It's said that Tobirama was the best water style user, but I wouldn't be suprised if he has been surpassed. Mei pretty much replicated what Tobirama did - water encampment pillar (he used wall) + water dragon with no hand seals, in an area with no water and to a much larger scale. Her water dragon dwarfed Madara's incomplete susanoo. Also, she can use boil release so therefore she must be a suiton master.
Its stated that Hiruzen was the most powerful hokage but after finding out more about Hashirama, people belive that he is the stronger of the 2. Hiruzen was the best in part 1 and Hashirama was best in part 2. This might be a similar case with Tobirama and Mei,