I've had only three different evolutions so far (Flareon, Vaporeon, Espeon) and all three were pretty good. Seems like you just can't make a mistake with Eevee.
Vaporeon. It clearly has the best/most inspired design. All the others are cute, but much more basic. Vaporeon really has an identity. The others are just "eevee but like, with some element related stuff on it!".