PHYSICAL SWEEPER MEGA MEWTWO X
Mewtwo is one of the original Ubers of Pokemon, being a gen 1 Pokemon, and being in Ubers since the dawn of Pokemon. Why? Mewtwo, is an extremely versatile Pokemon, and has amazing stats. With the Mega Stones, introduced in Pokemon X and Y, as Mewtwonite X and Y, further reason for it's justification in Ubers. I run Both Mega Mewtwo X AND Y. Mega Mewtwo has one of the best Attack stats in all of Pokemon, and that is what I will be focusing on. Mega Mewtwo X is weak to Ghost, Fairy, and Flying Moves. It isn't weak to Dark, as it is part Fighting. This Mega Pokemon is very versatile, and can be a mixed, special (not advised), or physical sweeper.
This is the move set I am satisfied with, and currently use on my Pokemon X Battle Box Team...
EV Spread: +252 ATK, +252 SPEED, +4 SP DEFENSE.
Drain Punch, To regain HP and Counter Dark type Ubers, like Darkrai, which is extremely common in Gen 6 Ubers. Due to Mega Mewtwo X being Psychic - Fighting, Drain Punch gets STAB.
Ice Punch, for type coverage, and countering Flying types. Oddly enough, it freezes Pokemon often enough to help me win.
Psycho Cut, STAB, and Crit chance. I use this over Zen Headbutt, as it has good Accuracy, buy Zen Headbutt has flinch chance.
Poison Jab To counter pesky Geomancy Xerneas with Power Herb, a common pest in Ubers. Poison Jab also has 30% Poisoning chance.
USEFUL SUBSTITUTE MOVES
Stone Edge, Zen Headbutt, Aqua Tail, Fire Punch, Thunder Punch, Earthquake, Rock Slide, Bulk Up, Aerial Ace.
Remember that Mewtwo in general is an extremely versatile Pokemon.
I wish that Mewtwo got a physical Dark or Ghost move. Foul Play isn't useful. I wish it got Knock Off, which is actually useful.
Mewtwo is one of the best sweepers ever :)