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Considering game mechanics, OHKOs, typing, usual sets and speed tiers, Charizard if it has access to Charizard X.
Resists or is neutral to their attacks, survives rock attacks (even STAB HP rock from Greninja) and can OHKO back. Mega Lucario's the only one that can OHKO with Close Combat but since speed doesn't apply on the turn it mega evolves, Charizard KOes first. If we apply the speed on the turn it megas, Charizard can remain at base form and OHKO with Flare Blitz but it kills itself half the time.
If no megas, Greninja. Faster than the others, survives priority and can OHKO. Typing isn't the only thing to consider since Pikachu even dies from Lucario's ES.
