Floatzel is our champion. It has the same base speed as Starmie (115 base speed), which it is tied with as the fastest water type. But what sets it apart is that it gets Swift Swim, which doubles its speed in rain, thus making it twice as fast as Starmie.
361 (Base Speed 252 EVs, + Speed Nature, 31 IVs) * 2 (Swift Swim boost) = 722
722 * 1.5 (Choice Scarf boost) = 1083
So it will have 1083 base speed with all the boosts applied. c: