My top three are these:
1. Espeon - Thanks to Magic Bounce, Espeon has solidified itself as the best Eeveelution of them all. Able to reflect any entry hazard or stat detriment as well as being able to use that base 130 Special Attack stat to its advantage makes Espeon an excellent Pokémon to use. Even without Magic Bounce, having access to Shadow Ball and Signal Beam helps correct Espeon's weaknesses to Ghost and Dark-types.
2, Jolteon - Back in the heyday of Gen 1, Jolteon was probably the most commonly used Eeveelution as it was a damn sight better than Raichu in every way. Base 130 Speed also meant firing Pin Missile's at Alakazam could often net a 2HKO easily. Nowadays, Jolteon still holds its own in the top tier as one of the premier Electric-types, and has a movepool that can both deal masses of damage and scout its opponents. It also fills the all-important BoltBeam combo with Hidden Power Ice. Oh, and did I mention it has access to Signal Beam? It's arguably in the same tier as Espeon.
3. Leafeon - Often overlooked in favour of other Grass-types, Leafeon is one of those Pokémon that can fill certain niches better than most of its competition. Like Sceptile of Gen 3, it's the premier Leaf Blade user, and with Chlorophyll as its Dream World Ability, can fill in the Physical void that other Grass-types simply can't, and is often a good foil for Venusaur or Exeggcutor.
The only problem with Eeveelutions on the whole is that their movepools are limited, but if you can play to their strengths, they'll always serve you well, except Flareon, because Flareon has nothing good to offer. Until Game Freak give it access to Flare Blitz, that Attack stat it has is wasted, thus Leafeon is a far better choice overall.