I hope that they will have a DLC Mega Stone for Eevee, allowing him to Mega Evolve into Mega Eevee. It is the best way to give him a Normal evolution. They can even make his ability be Protein, to give all his attacks STAB, and show off his elemental instability.
I haven't actually used half of them (Vaporeon, Flareon, Glaceon, and Leafeon, to be precise) before, but of the ones that I have had in a team, Jolteon's got to be my favourite. I never had as much of an affinity for the Eeveelutions as most people seemed to, but I always thought Jolteon looked the coolest. I also thoroughly enjoyed using it in-game, though I suppose my playstyle tends to favour fast, hard-hitting Pokes.
Vaporeon is my favourite Eeveelution. Every generation I tend to try out the new Eevees and I see myself continously coming back to Vaporeon. Espeon as well, but there can only be one number 1!