It depends. Competitively, Jolteon and Vaporeon are more widely used; Vaporeon has amazing HP, and a great Sp. Attack, while Jolteon is faster, with high Sp. Attack. Really, it all stands to interpretation of how people use them. Leafeon could be brilliant, if it had a wider range of physical attacks that are its type, (Leaf Blade is good, and all, but its pretty predictable, and Razor Leaf is just a pain, with its 95% accuracy (likable to Flareon's only physical Fire type move; Fire Fang)) and oh; look at that. Leaf Guard is great, too. Just make sure you take out any Fire types before doing that tactic; Leafeon will fall quickly, if they're still around.
I'd actually say Umbreon is pretty good, as well, since it has great defences. Can't go wrong with a bit of defensive play, right?
I'll stick to Glaceon, though; she's a personal favourite of the Eeveelutions, for me, and she's got pretty good Defense and Sp. Attack. If you get a hailstorm going, Snow Cloak can be your biggest ally, as well.
If you can handle Flareon, it can be good, as well, but its a little weak, with the lack of same-type moves and the low defense/HP.
Really, its all up to opinion and preference. But you get a good idea from asking, so it never hurts. XD