I believe it to be Tobirama but evidence states its Kisame. Evidence>My opinion so Kisame..honestly if mei had the chakra levels of kisame she would take it due to her chakra natures and the ability to fuse them with suiton. this shows she has better chakra molding skills than Kisame. all she needs is more chakra to be in this debate aswell but too bad shes not.
Remember if you walk into the Ward of Dawn you ain't walking out
Re: who do you consider the beSt water user?
Tobirama was revered and known around the world as the best suiton user. I believe Tobirama is still the best considering he can use suiton much more efficiently compared to Kisame.
For example Kisame takes multiple seals to create a shark bullet (ten according to the wiki) with a water source present, whilst Tobirama can make a water dragon out of thin air using one seal. Water dragon and shark are similar techniques just with different shape manipulation applied.
So Tobirama has large scale suiton that can be activated with minimal seals and no external water source - placing him first on the list. Kisame has very large scale suiton that can overwhelm opponents making him second. The 2nd Mizukage only uses hozuki clan jutsu soo he would be 3rd.