(awesome sprite by the way)
Dragonite @ Leftovers / Lum Berry
Ability: Multiscale
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Dragon Dance
- Dragon Claw
- Fire Punch
- Roost / Earthquake / Extreme Speed
In order to steer away from the most commonly used cores with Water and Electric types, I'm going to nominate Dragonite.
Defensively, they share a great synergy, providing resistances to 15 out of the 18 attacking types (the others being Electric, Dark and Ghost), with only one overlap being Grass. This means they can easily get in the field, and the fact that these resistances are well spread between the two means they can often create opportunities for the other to come in as well, which brings us the offensive aspect of the synergy.
Metagross-Mega is excelent in paving the way for a potential Dragonite sweep, luring and weakening Pokémon such as Heatran, Landorus-T and Rotom-W, and being one of the most reliable Fairy Pokémon killers in the game. Also, Metagross-Mega can often bait Ground-type moves (many times locked) which Dragonite can gladly set up on.
If the goal is instead to achieve a Metagross-Mega sweep, Dragonite can severely dent or outwright muder pokémon such as Skarmory, Scizor and bulky Water or Ground-types, including the aforementioned Landorus-T and Rotom-W.
Overall, they provide two win-cons for the team, all the while mutually helping each other and keeping offensive momentum on the core user's side.