Gardevoir
It has a better typing, and is an amazing special attacker. Most of the time speed isn't an issue in playthroughs.
Gardevoir @ Twisted Spoon (Found as a deal in Stow-On-Side)
Recommended ability is Synchronize and Modest nature
-Moonblast (Move Relearner)
-Psychic (Learnt at level 42)
-Shadow Ball (TR, get it from Gastly raids at the very start)
-Thunderbolt/Calm Mind (Thunderbolt is a TR and Calm Mind is learned at level 35)
You can find a Ralts in Rolling Fields, South Lake Miloch, Dappled Grove, West Lake Axewell, North Lake Miloch, Watchtower Ruins, and Fields of Honor.
With these moves, Gardevoir can have supereffective damage against:
Gyms
Nessa's entire team
Allister's entire team
Piers' entire team except for Skuntank
Wyndon Stadium
Nessa's entire team except for Quagsire
Allister's entire team
Piers' entire team
Evil Team
Basically all of the Team Yell grunts' Pokemon, since they use Dark-types
Rivals
Hop's Inteleon*, Corviknight, Cramorant, and Trevenant.
Marnie's entire team
Bede's entire team until Wyndon stadium
Champion
Leon's Charizard, Inteleon**, Haxorus, Dragapult, and Aegislash.
It also gives you powerful neutral damage, and coverage for tons of random trainers.
*If you choose Grookey.
**If you choose Scorbunny.
