Sylveon
- Solid HP, Special Attack and Special Defense
- STAB attacks super effective against Korrina, Drasna, and part of Diantha's team
- Learns moves like Shadow Ball, Moonblast, and Hyper Beam
- Mediocre Defense and Speed
- Shallow movepool
- It can be a pain to evolve Eevee
Gardevoir
- Amazing Special Attack and Special Defense
- Respectable Speed
- STAB attacks super effective against Korrina, Drasna, and part of Diantha's team
- Learns moves like Moonblast, Thunderbolt, Psychic, Shadow Ball, and Energy Ball
- Can Mega Evolve
- Large and varied movepool
- Ralts is a bit tricky to find
- Iffy HP and Defense
Gallade
- Gargantuan Attack and solid Special Defense
- Respectable Speed
- STAB attacks super effective against Grant, Korrina, Wulfric, Wikstrom, and a bit of Dinatha's team
- Learns moves like Close Combat, Leaf Blade, Brick Break, Earthquake, X-Scissor, Poison Jab and Psycho Cut
- Massive movepool
- Can't get a Dawn Stone until later in the game
- Iffy HP and Defense
I would go with Gardevoir or Gallade.
Gardevoir @ Expert Belt / Pixie Plate / Twisted Spoon
Ability: Synchronize
- Psychic
- Moonblast
- Thunderbolt
- Energy Ball / Shadow Ball
Gallade @ Black Belt / Twisted Spoon / Expert Belt
Ability: Steadfast
- Close Combat / Brick Break
- Psycho Cut
- Poison Jab
- Earthquake
Hope I helped!