My first time through the game, I didn't evolve Eevee at all, because I could never figure out what I wanted.
The first question is what would fit best in your team. Which do you need most? Fire, water or electric?
You might actually try out all three of them and just see which one you like best. If you already have all three of the stones needed to evolve them, then just make sure they're in your inventory and save the game. Otherwise, go to the Department Store and go up to the counter where they sell the stones and save it there. Then either pull a stone out of your inventory or buy it from the store, use it on the Eevee, let it evolve and see what you get. I'd take it out and battle with it a bit, just to see what it looks like and how it handles itself in combat. Then reset the game and do that again with the next stone and the next evolution. Keep doing that until you've tested all three and see if that leads you to choose one over the other two.
In my own opinion, Vaporeeon is the best, Jolteon is a close second and Flareon is a distant third, but that's just my opinion. What really matters is which one would fit best on your team and just which one you like the best.
Hope that helps....