Vaporeon!
A little of it comes from just plain love for Water type Pokémon, and its just so graceful in the way it moves, and, I dunno... I can't explain, but Vaporeon just owns.
Flareon and Jolteon seem to be a favourite of a lot of people, but Jolteon can only use either Normal or Electric attacks (at least, most of em anyway) and Flareon... well, lets just say I didn't have the best luck with it (I had trouble with it in the game) and its too... favouritey.
I mean, almost all fans will say "OMG I LOVE FLAREON!" or something, though I may be wrong. I just prefer Vaporeon. *hugs Vaporeon*