Eeveelutions are my favorites in general, and it's hard to go wrong with whatever one you pick.
That said...
Jolteon and Espeon both make for good sweepers- they're both speedy and have strong sp. attack but have a hard time surviving physical attacks.
Umbreon can take a lot of hits, but doesn't attack very hard. Makes a great supporter, though.
Sylveon and Leafeon specialize in special and physical atk/def, respectively.
Flareon's a good attacker, but since it isn't specialized, its good sp. atk is wasted next to its far better physical atk.
Vaporeon strikes me as, y'know, fine. It can hit, and it can take hits, but neither of them are too remarkable.
Glaceon has fantastic sp. atk and great defenses... with an absolutely terrible moveset and significant type weaknesses.
so you should get an all eeveelution team that maybe doesn't have flareon or umbreon.