I love Eevee, but my favorite Eeveelution is Espeon.
First, the design is cute. I like cute Pokemon.
Second, its Psychic type, and Psychics are one of my favorite types.
And most importantly, to moveset is so powerful! If you train Eevee to learn "Bite" (my personal favorite dark move), then evolve into Espeon and learn "PsyWave", you have a Pokemon that knows both Psychic and Dark moves!
In my Crystal version, my Espeon could take out almost all of the Elite Four! ^_^