Others have said before that Gyarados and Floatzel is an unfair match-up, which I do agree with, and even so, I'd still have to go with former.
Gyarados is a very powerful Pokémon in itself, generally; it has access to Dragon Dance, and after one use of it, it'd be gaining instant boost in Attack and Speed, immediately setting up the Pokémon for sweeping. I've used this strategy with it dozens of times before, and it was always great.
Floatzel is not so bad, either. Personally, I feel its design is unique; when I'd first seen it, I thought it was normal-type, and was surprised to see it to wind up to be water. I enjoy twists like this every now and again.