Actually, Ash-Greninja fought Toe-to-Toe with Alain's Mega Charizard X, who
easily blocked a heat wave from Trevor's Mega Charizard Y in Base, which was boosted by drought. That same Mega Charizard X also
one shotted this Mega Charizard Y with a flamethrower
So: Alain's Mega Charizard X >>> Base Charizard >> Trevor's Mega Charizard Y with drought >> Mega Charizard Y >>> Base Charizard
Ash Greninja < Alain's Mega Charizard X, but only slightly, as they were almost on equal footing for a long time in their match
Both Ash's Greninja and Alain's Mega Charizard X fought toe to toe with the Champion's Ace Pokemon (Charizard against Steven's Metagross, and Ash against Diantha's Gardevoir)