I would pick Vaporeon and Flareon.
First off, water and fire types just own. They just do.
Second, Vaporeon already has the advantage to Rock, Ground, Fire, and is resistant to Ice and Steel.
Flareon has the advantage over Steel, Grass, Bug, and Ice. Vaporeon is immune to water, and Flareon is immune to fire.
You got 7 type advantages right there.
Eevees learn Bite, so there's an advantage against Psychic and Ghost.
Either one can learn Dig, so there's cover for Electric and Poison.
Vaporeon can learn Ice moves, so there's cover for Dragon.
Either one can learn Iron Tail, so there's cover for Flying.
And either one can learn Rock Smash, so there's cover for Normal.
Really the one type they couldn't have advantage over would be Fighting.
And both of them have very good defense usually.
And I'm sure they could take out a Fighting type easily without having an advantage (as long as they have good moves and stats).
So there ya go.... Flareon&Vaporeon win. To me, anyway. And based on all this stuff...they really would make a good team.(: