Chandelure
It has ridiculous Special Attack, and respectable defenses. It isn't the fastest, however. You can catch and evolve a Litwick very easily, and get a Chandelure almost immediately after arriving in Lentimas Town. It learns moves like Energy Ball to cover its weaknesses, and strong STAB like Flamethrower and Shadow Ball. It's super effective against Shauntal's and Caitlin's teams, and a bit of Iris' with STAB.
Volcarona
It doesn't get many good moves until later levels, meaning it's deadweight until then. You can get it earlier, but that doesn't amount to much when it doesn't learn many good things. Its Special Attack just barely falls behind Chandelure's, but it has better Special Defense and Speed to make up for it. It's also time-consuming to find it in the Relic Castle. Its STAB moves are super effective against Grimsley, and a bit of Iris' team.
I personally prefer Chandelure, but I'll provide sets for both, as they're both good.
Chandelure @ Spell Tag / Charcoal
Ability: Flash Fire
- Flamethrower
- Shadow Ball
- Energy Ball
- Will-O-Wisp / Psychic
Volcarona @ anything
Ability: Flame Body
- Flamethrower
- Signal Beam
- Psychic
- Fly / Quiver Dance
Hope I helped!