Yet, I can firmly say that as Ash-Greninja, he is arguably Ash's most powerful Pokémon. Without his transformation, I could still stay he's in the top tier of Ash's strongest team. He's been able to handle plenty of different opponents before utilizing the transformation, which occurred at random at first because of Ash and Greninja's hearts and minds syncing up.Ash-Greninja is not his most powerful pokemon. It is Pikachu and his Charizard that are his best pokemon. Followed by Sceptile, Infernape and then Greninja.
Even now, it still does. Ash and Greninja just go about the transformation more or less naturally as the battle goes on. It's their version of a Mega Evolution.