Haxorus
- Incredible Attack and excellent Speed
- Respectable Defense
- STAB attacks super effective against Drayden and part of Iris' team
- Mold Breaker deals with the countless annoying Sturdy Pokemon
- Learns things like Aqua Tail, Dragon Claw, Shadow Claw, Brick Break, X-Scissor, and Poison Jab
- Evolves late
- Merely okay HP and Special Defense
Gigalith
- Excellent Attack and Defense
- HP and Special Defense are pretty good as well
- STAB attacks super effective against Skyla and part of Iris' team
- It being a traded Pokemon means boosted experience
- Learns moves like Iron Head, Stone Edge, and Strength
- Incredibly slow
I would go with Haxorus. Here's a set:
Haxorus @ Dragon Fang
Ability: Mold Breaker
- Dragon Claw (Level 28)
- X-Scissor (TM found on Route 7)
- Return (TM found in Aspertia City)
- Dragon Dance (Level 32)
Hope I helped!