Well, the best wall in ubers is Lugia.
Lugia @ Leftovers
EVs: 252 HP, 176 Def, 80 Sp. Def
To be honest, Lugia is probably the only thing that can safely deal with Mewtwo without taking into account what set the Mewtwo is running, or what mega stone it's holding. That being said, Lugia can only phase, it can't do a lot to Mewtwo except tank its hits. If you want a counter, use Sucker Punch Mega Absol.
However, even if the Mewtwo is packing Mewtwoite X, it'll still most likely run a mixed set, considering its abundance of reliable special moves that provide coverage.