for me, the 4 best suiton users in the series are:
Tobirama Senju -- he may not have a biggest suiton-based feat, yet he certainly has the most advance.. we are yet to see another shinobi who can create water and perform such a high level suiton jutsu from the water molecules in the air.. both Kisame and Mei have yet to demonstrate such feat.. Kisame created such a large scale water jutsu but that water was from his mouth which basically create out of chakra.. and we all know that it takes much more ability to create the water outside the body than to manipulate what is already available or expel it from their mouths.. also, one cannot say that Tobirama can't create a water based jutsu as large as Kisame did since during his fight with Hiruzen, they are confined in the Four Violet Flames Battle Encampment, so determining the possible extent of his suiton jutsus are basically impossible.. but for raw skill in manipulating water based jutsus, i pick Tobirama as the best..
Kisame Hoshikagi -- he has displayed the most powerful water based jutsus yet based on magnitude and size.. this is probably connected to his large chakra reserves.. i have to admit that the feats that he has shown with suiton jutsus are greater in magnitude than that of Tobirama, but this is probably because, he has shown more jutsus that Tobirama but that doesn't prove he has more skill that the Second Hokage..
Mei Terumi - she has displayed the greatest feats in suiton jutsus save the two before her.. her skill is also worth noting as she is the only one shown capable of creating a high level defensive suiton jutsu into a high level offensive suiton jutsu.. she has managed to counter most of Madara's fire based techniques on her own despite proving in panel that it takes almost an army of regular suiton users to counter just one of Madara's fire jutsu..