I know this is about Mega Charizard X, but in a realistic situation, you don't know what kind of Charizard you'll be facing, so I think the best checks are going to be the Pokemon that can deal with both Charizards. Since Mega Altaria isn't released (which is a shame, I feel like that thing might be better than it was last gen lol), that leaves us with the 3 Scarfers: Nihilego, Garchomp, and Terrakion! Honestly I think all 3 are good choices... I think I'll vote for ThickFatAzumarill's Nihilego just because I appreciate that it's a scarfer that can straight up clean up teams with Beast Boost, has a 100% accurate move, and it doesn't have to worry about the rare Will-O-Wisp. Terrakion may be a bit better in other ways however, because I like how it deals with those Ferrothorn switch-ins that plague the other two, and nailing Porygon2 is mad cool for a scarfer as well. Really, I don't think you could go wrong with any of the 3!