It depends on what you want. Espeon's a good special sweeper, Umbreon's a trap-passer who laughs at many physical attackers, Vaporeon's a special attacker/tank/Baton Passer, Jolteon's just a Dugtrio killer, and Flareon... Well, let's just say few battlers even bother with Flareon.
Personally, though, I like Umbreon. Though it's highly predictable (seeing as pretty much the only thing it can do decently is trap and Baton Pass), it's still the best Eeveelution for that job.
Standard set goes as so:
Bold Umbreon @ Leftovers
(Synchronize)
- Mean Look/Curse
- Taunt
- Baton Pass
- Moonlight/Wish
Above is pretty much what you'll see most of the time. People either use Mean Look to perform your basic trap-pass, or they'll opt for passing the stat boosts (and, okay, the Speed decrease) with a Curse + Baton Pass combination. Taunt is for the pHazers (naturally). Moonlight is for healing up, especially to survive stat boosting if you're going with stat passing, although Wish is nice too. (I've heard it works better, but I'm the sort who prefers instant results when it comes to healing moves.)
Good luck!
Edit Note: Yes, many team building gurus will probably tell you that Umbreon can pull off Baton Passing better than Espeon, although it's debatable when it comes to Umbreon vs. Vaporeon. Espeon is usually used as a special sweeper (or, in Smogon's opinion, tank), although it's sometimes used to pass special stat boosts with Calm Mind and Baton Pass. Really, there's no point in passing, in my opinion, given the fact that Espeon has the movepool and stats to just Morning Sun and waste the opponent with Psychic/Hidden Power after boosting its own stats. It's not quite as awesome as Alakazam or Gardevoir in that department, but hey, with 130 SA, no one's going to question what that baby can do.
Unfortunately, if you've already got a special sweeper/tank on your team, you might as well go with Umbreon. I personally prefer Umbreon more for the simple reason that I'm evil.