Flareon will get the ability Guts
When this Pokemon is affected by burn, paralysis, poison, or sleep, its Attack is increased by 50%. Guts ignores burn's Attack drop.
So, if Flareon holds a Toxic Orb or gets statused, it gets an attack boost! Cool!
Vaporeon will get the ability Hydration
When Rain Dance or Drizzle is in effect, this Pokemon heals itself of burn, freeze, paralysis, poison, and sleep at the end of each turn. If rain ends on the same turn that this Pokemon is afflicted with a status, that status will not be healed.
So, if Vaporeon is in the rain, she cannot be statused! Awesome!
Jolteon will get the ability Quick Feet
When this Pokemon is afflicted with a status condition, its Speed increases by 50%. Furthermore, the Speed drop of paralysis is nullified (but the chance of being "fully paralyzed" is not).
Yet another status ability! Similar to Guts, but you get a Speed boost instead.
Espeon will get the ability Magic Bounce
Reflects any moves that change status effects (bar Memento), status conditions, entry hazards, Taunt, Encore, Disable, and all other pseudo-conditions, and provides an immunity to said effects. If both Pokemon have Magic Bounce, only the ability of the target triggers. Magic Bounce only activates once per turn. Teravolt, Turboblaze and Mold Breaker all negate this ability
What a cool ability! Instead of being immune to status, or getting boosted by it, the status gets bounced back! Wow!
Umbreon will get the ability Inner Focus
Pokemon with this ability cannot be flinched. Does not prevent the turn loss if the user is attacked while charging Focus Punch
Cool! Say adios to flinching! So, no more Fake Out...
Leafeon will get the ability Chlorophyll
This Pokemon's Speed is doubled under sunlight. If the sunlight ends or a new weather condition is brought into play, the Pokemon's Speed returns to normal.
Wow! In the sun, Leafeon becomes twice as fast! Amazing!
Glaceon will get the ability Ice Body
When hail is in effect, this Pokemon heals 1/16 of its max HP at the end of each turn. In addition, if a non Ice-type Pokemon has this ability, it will not take damage from hail.
Cool! Basically an extra Leftover healing!
So there they are. Pick whichever one you like; I recommend Espeon.