Genaller
Silver Soul
THIS!We don't know how strong he is after his training yet you have XYZ's fanboys knowing everything without concrete evidence about that matter.
Well this reasoning is loose but bear with me. Charizard trained in the CV with the goal of being the strongest of its species when initially it couldn't beat any of the other Charizard their. At the end of BW, Charizard is implied to have not returned to CV implying he has nothing left to do there meaning he's surpassed everyone in CV placing him among the top Charizard in the planet. Alain's Charizard can keep up with and later on even beat E4 Aces (since as MC X it can beat E4 Megas). E4s are defined as master trainers of a singular type meaning their Pokémon should be amongst the strongest of their species. Alain's Charizard being on that lvl must mean it's also amongst the strongest Charizard in the world. Since both Charizard are amongst the best of their species, chances are that they're on par. So a statement like Ash's base Charizard ~ Alain's base Charizard while not iron clad is still quite reasonable. Also the first thing everyone thought of when Alain was introduced was Charizard vs Charizard (and not the lame version we got in the KL). I don't think Charizard is Ash's best (3rd for me) but I will admit that Ash's Charizard is probably the most 3-dimensional Pokémon in the anime.