Mega Mewtwo X -> B
Mega Mewtwo X is actually really underused, and mega mewtwo Y is alot more common, but i believe that mega mewtwo x is really good. I run a max atk adamant nature, which, though it leaves mega mewtwo x at 359 speed, breaks through a ton of pokemon, especially with either sticky web on the opponent, having a life orb mashadow on your team, or both. It has a good move pool, and can run rock tomb, poison jab, and/or fire punch to counter common threats or pokemon in the battle. Though scarf yveltal would probably murder it with oblivlion wing, mega mewtwo x is extremely solid, and can usually switch into a hit against most pokemon, as it gets decent bulk of 100 in atk and defense.
If you really wanted to be cheeky, you could even run a mixed set with mega mewtwo, as the base 190 only needs a little support, and you could run special with around 160 put into it, though i've never actually tried to run this.
mega mewtwo x, in my team, usually cleans up the opponents, as it can usually one-shot most pokemon at 50% or lower. it counters ferrothorn with fire punch, xerneas with poison jab, and can keep dd mega sala in check with rock tomb, usually letting marshadow switch in once mega mewtwo dies, forcing a switch. not only that, but since everyone expects mega mewtwo y, its use of physical attacks is usually unexpected, and can lead to wrong switches into special tanks, such as chansey, letting it die to a drain punch.
IDK, this is my opinion on mega mewtwo X, and i totally get it if you don't agree with me.