In-game-wise: I used only Vaporeon, Flareon and Espeon and they all were very very good. But I've never used them in the same game, not even the same generation so I can't really tell which one was the best one. Maybe Vaporeon.
I'm still slightly pressed they made Glaceon. Like what a waste...we already have a potentially ice-type (Vaporeon). They should have made a rock/ground/steel type.
Electric: Jolteon 
Fire: Flareon 
Psychic: Espeon 
Grass: Leafeon 
Ice: Glaceon 
Fairy: Sylveon 
