While on the topic of the Mega Charizard's I guess I'll speak about it. Just to let you know I was one of those kids that loved him and chose him as my first starter, so of course my opinion will be bias haha.
Charizard Y will most defiantly be OU imo. We've already seen what affect Drought had on Ninetales, turned an utterly useless pokemon into a staple in the OU metagame. Now Charizard Y will most defiantly be a threat its self, having a way better movepool than Ninetalesand most likely way higher Special Attack and probably speed making it a threat it's self in sun. But that's not the main draw to it imo, the most interesting thing about it is it has access to instant recovery in the form of roost, which no other weather inducer does (except hippo of course) giving it a niche over Ninetales. This straight up makes it interesting and nullifies its SR weakness to an extent. Think about it, he can switch into something that he directly threatens like Ferrothorn, take damage from SR and pitiful damage from Ferro's powerwhip, then roost up when Ferro switches out. The biggest problem for him will be of course his SR weakness, which we cant ignore is a huge problem, but having access to drought, being more threating and having access to instant recovery will give it a niche over Ninetales and imo place him in OU. I should also mention this now but he will probably be the better of the two Mega's competitively anyway.
The biggest problem for Charizard X will be he at least has to switch in as Fire/Flying type once per battle, meaning he will take 4x damage from SR if they are in play. But this can also be used to his advantage, not taking the damage from SR obviously but using his flying type in a positive way. For a start he wont have to take spike and toxic spike damage, but he can also switch into EQ without taking damage. The more interesting to me though is it can bring a whole new element to the game. On the DS it shows a button below that you press when you want to mega, suggesting you don't have to mega evolve. This means you can potentially predict if someone's going to go for EQ to hit your Charizard X for SE damage and instead you set up a DD, or you can set up on EQ locked pokemon. Anyway apart from this Charizard X should be a threat. Fire/Dragon is great typing and good STAB, he has a great physical movepool to go with his attack increase, a great ability further boosting his physical strength and has great set up moves in SD and DD. This guy will be viable in OU imo.