RE: Pokemon XD: Gale of Darkness- Eeveelutions
It really all comes down to preference, based on what your team is, and what you want the Eeveelution to do. Jolteon and Espeon are probably the best, but here's what each one does:
Vaporeon - Strong points are HP and Def. Being Water-type, it can learn strong moves from both Water and Ice types. It can also use Acid Armor a few times, and then Baton Pass it. Surf hits both opponents in a 2v2 so that might be a good choice.
Jolteon - Strong points are Speed and Sp. Attack. One of the fastest Pokemon out there, Jolteon can be a total monster as it rains down Thunderbolt, and it's virtually guaranteed to outspeed most other Pokemon.
Flareon - Strong points are Attack and Sp. Defense. It can do okay with Flamethrower, and wall against quite a few special attacks, but aside from Shadow Ball and Return, its physical movepool is rather weak.
Espeon - Strong points are Sp. Attack and Speed. Whereas Jolteon has its forte in speed, Espeon has its forte in special attack. It's mostly the same as Jolteon, except it's got the option of Psychic and a few other things at its control.
Umbreon - Strong points are Sp. Defense and Defense. Although pretty much no other Pokemon in Gen 3 can wall like Umbreon, its physical strength is rather weak. Then again, you could stall out your opponent forever with what it can use.
I'm not sure of the TM availability and all that in Gale of Darkness, but given what you know you can find in that game, make the best choice for yourself.