This is a tough one
-Vaporeon has amazing HP and Spa Attack
-Jolteon has monstrous speed and Spa Attack
-Flareon, has the highest psychical offensive stats of the Eeveelutions, but Flareon needs a new psychical Fire type move, it's my favorite Eeveelution, but Fire Fang isn't cutting it
-Espeon same as Jolteon
-Umbreon, Amazing defenses, and I'd say Umbreon has the coolest design of all the Eeveeltuions, especially when it's shiny
-Leafeon, awesome Defense and Attack
-Glaceon, fantastic Defense and Spa Attack
So hard to choose, but, I think it's Espeon and Jolteon, since speed plays an important part in battles.