Espeon is probably your best option. In-game is all about hyper offense, so you want the eeveelution with the best attacking stats. Flareon has great physical attack, but it's slow and has a bad movepool, which is why I didn't mention it. Jolteon is also great, but has type overlap with Vikavolt. Sylveon is also a good option, but Espeon does pretty much everything Sylveon does but better.
Espeon has a great special attack stat, with 130. It also one of the best movepools of the eeveelutions with moves like Shadow Ball, Psychic, and Dazzling Gleam. Another thing Espeon has going for it is it's great 110 speed stat, making it the fastest evolution of Eevee save Jolteon. Espeon is usually a pain to evolve, but the introduction of curry makes it no problem at all.
Here's a moveset to have before the post game:
Espeon @ Twisted Spoon (or whatever you want)
- Psychic (by TR) / Psybeam (level up)
- Dazzling Gleam (by TR)
- Shadow Ball (by TR)
- Calm Mind (TR) / Work Up (TR, if you can't get Calm Mind)/ Morning Sun (level up, but Potions do it better)
You can also use something like Swift or Hyper Voice in the last slot. Make sure it doesn't have a speed or special attack decreasing nature.