I could never get into Flareon, cause I always use my Fire Stones on Vulpix. I've never played the 4th generation games, so that rules out Glaceon and Leafeon.
Of the 2nd generation ones I prefer Umbreon, cause the first time around when playing Silver, that's what my Eevee evolved into and I fell in love with it and as such, I became a fan of the dark type. The second time I played I got Espeon, but it wasn't the same.
That leaves Vaporeon and Jolteon. I always liked them both in Pokemon Stadium, Jolteon seemed so bad-ass, but there was an elegant quality about Vaporeon that made me fall in love with it, plus, its stats and aurora beam and acid armour etc.
SO: Vaporeon and Umbreon would be my favourites. After that it goes: Jolteon, Espeon, Flareon, Glaceon, Leafeon.