Vaporeon was always the best to me, with its good movepool and stats.
jolteon is really fast but doesn't seem to have as much special attack or HP as vaporeon.
Flareon has a terrible movepool and more attack than special attack, so it was worthless before D/P/P.
Umbreon has great defense/sp. defense stats, but thats all it has.
Espeon has the best sp. attack of all eeveelutions, but it has low defense and can get slaughtered by a bug move.
glaceon just doesn't seem to be my type because its new and one of the few ice types.
Leafeon has a lot of attack, but it seems like thats all it has.
and that is why i think vaporeon is the best of all others.