It most likely be a misconception as Gold Lightning said & I said both Kisame & Mei are better than him so I don't see the point of this discussion.
It was a mistranslation from the first Databook made many years ago. And ever since then, people have believed it to be true.
Truth is, there is no such statement that hypes Tobirama up to be the best in suiton. This is all the databook says:
He was a superior ninjutsu user, being able to use Water Release without being near the waterside, among many other feats