I'd say Vaporeon, good water attacks & attack it can be taught. Rain Dance, Hydro Pump, Ice Beam, Blizzard, Shadow Ball etc...Don't pick Umbreon though, I did & not that good. Fantastic defence but not that good attacks. My Umbreon's attacks are:
Phychic (by TM)
Quick Attack (good for getting rid of little health & if you're about to be knocked out, at least attack first)
Confuse Ray
Mean Look
Not that good, want to get rid of Quick Attack though. Faint Attack pretty good but got rid of it for Phychic. Does 60 damage but want something more powerful. Might get rid of Mean Look because I don't think opponant ever retreats their Pokemon so what's the point holding them there...?
Jolteon is okay but Flareon a good choice also. Espeon I dunno what attacks it has, I forgot. I have obviously both Espeon & Umbreon on Collosieum, but dunno...
Remember though, whichever you go for, don't train another of that type. You want different types in order to win!