It comes down to either tobirama or mei (even though she isn't in the poll). Kisame is over-rate and yes - his water style jutsu may be of a large scale but I think this has more to do with his huge chakra reserve as opposed to skill.
Tobirama was revered as the greatest water style user because he could perform powerful water style jutsu in areas where there was no water. He was also able able to perform the water dragon bullet jutsu with one hand sign as evidenced by the image below.
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However, mei was able to replicate this feat with her fight against madara uchiha which is evidenced by the image below. I'd also like to note the huge scale of her jutsu.
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I wouldn't be suprised if mei's skill in water style jutsu was on par, if not greater than tobirama - she can use boil release which is a combination of water and fire elements, so she probably is a master at water style jutsu.