Dialga

The best possible counters, at level 50 with perfect IVs.

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1. 1 Mewtwo (Mega X) Mega Counter Elite / Focus Blast 36 Estimated DPS
2. 2 Lucario (Mega) Mega Force Palm Elite / Aura Sphere 34 Estimated DPS
3. 3 Blaziken (Mega) Mega Counter / Aura Sphere 35 Estimated DPS
4. 4 Groudon (Mega) Mega Mud Shot / Precipice Blades Elite 31 Estimated DPS
5. 5 Mewtwo (Mega Y) Mega Counter Elite / Focus Blast 31 Estimated DPS
6. 6 Heracross (Mega) Mega Counter / Close Combat 28 Estimated DPS
7. 7 Garchomp (Mega) Mega Mud Shot / Earthquake 28 Estimated DPS
8. 8 Landorus (Therian) Mud Shot / Sandsear Storm Elite 27 Estimated DPS
9. 9 Alakazam (Mega) Mega Counter Elite / Focus Blast 27 Estimated DPS
10. 10 Lucario Force Palm Elite / Aura Sphere 26 Estimated DPS
11. 11 Mewtwo Shadow Counter Elite / Focus Blast 27 Estimated DPS
12. 12 Mamoswine Shadow Mud-Slap / High Horsepower 27 Estimated DPS

Ranked by estimated cycle DPS (type effectiveness, STAB, the shadow bonus and the target's defense). Not a battle simulation: no dodging and no survivability, so a fragile Pokémon can rank higher here than on other sites.