Mewtwo @ Expert Belt / Leftovers
Timid Nature
EVs: 252 Sp.Atk / 210 Spd / 48 HP
- Calm Mind
- Shadow Ball / Ice Beam
- Shadow Ball / Aura Sphere
- Taunt
Taunt is for stopping others from attempting to set up on Mewtwo (PHaze, status problems, etc). Both Ice Beam and Shadow Ball deal damage to a lot of other Ubers in the metagame, Aura Sphere is mainly added here to hit Blissey harder.
Mewtwo @ Choice Specs
Timid / Modest Nature
EVs: 252 Sp.Atk / 210 Spd / 48 HP
- Aura Sphere
- Ice Beam / Shadow Ball
- Grass Knot / Thunder
- Flamethrower
This Mewtwo does serious damage to anything, and outspeeds everything expect for Deoxy-F and other Choice Scarfers. If you're playing with other Ubers, then Grass Knot hits both Groudon and Kyogre hard. Aura Sphere is an OHKO on Darkrai.
From your other thread, I could see you're having troubles with obtaining a Timid natured Mewtwo, but this set can also work with Choice Scarf, so it shouldn't be too much of a problem switching to Modest nature.