Mewtwo has the Nasty Plot of world domination
Mewtwo @ Life Orb / Expert Belt
Timid nature
Ability: Tension / Pressure
252 Sp. Atk, 252 Speed, 4 HP
-Psystrike
-Ice Beam
-Shadow Ball
-Aura Sphere
This Mewtwo functions best when baton passed extra Sp.Atk. Psystrike is powerful STAB that hits anything that isn't hit Super effectively hard, with its effect to hit on the physical defence. Pretty much everything that isn't hit super effectively with this set has lower Def than Sp.Def, namely Ho-oh, Kyogre, Reshiram, Chansey, Blissey, (pink blobs fear Psystrike more than Aura Sphere) and Palkia, and it also catches Blaziken, Tentacruel, Terrakion, and Heracross with super-effective damage, as well as hitting Calm Mind Manaphy on the physical defence stat. Ice beam hits Rayquaza, Dragon Arceus, Lati twins, Giratina, Lugia, Groudon, Garchomp, Thundurus, Zekrom, Shaymin-S, and Exeggutor hard, Shadow Ball hits Wobbuffet, Shedinja, Mewtwo, Deoxys, and Ghost Arceus hard whilst Aura Sphere hits Dialga, Ferrothorn, Tyranitar, Darkrai, Heatran, Kyurem-W, Kyurem-B, Cloyster, and Steel, Normal, and Dark Arceus hard, and achieves perfect neutral coverage together with Shadow Ball.
This set gets so much Super effective coverage it is just downright scary, especially with the ability to switch between moves and the insane speed. It hits almost the entire Uber tier for Super Effective damage. A Timid nature helps Mewtwo outrun Timid Darkrai, Timid/Jolly Arceus, Jolly Shaymin-S, and Jolly scarf Tyranitar. After Stealth Rocks and a single Nasty Plot pass Mewtwo OHKO's almost the entire Uber tier, including Lugia by using Ice Beam, just be aware of Scarfed revenge killers and CB Rayquaza or Attack Deoxys. Only Giratina and Defence Deoxys can hope to survive after Stealth Rock and Nasty Plot.
Tension blocks berry use of the opponents, which means you can hit Rayquaza and Garchomp through their Yache berries for example, but is quite situational, as berries are very uncommon in Ubers. Pressure is on the other hand also not that good for this Mewtwo, as it is designed to outrun and OHKO pretty much everything the foe throws at it before they can even move, and that makes Pressure fairly useless.
Still, this Mewtwo basically does what Mewtwo does best: Special sweeping. With an insane Sp. Atk stat, good Speed, and a ridiculously large special movepool that allows unheard of levels of super effective coverage Mewtwo is the epitome of a fast special sweeper. Whilst it can do other things, Special Sweeping is by far the best thing Mewtwo can do.
Other options: Thunder, Flamethrower
Thunder hits Kyogre hard and hits Lugia a bit harder than Ice Beam would, but if you sacrifice Ice Beam for it you lose a huge amount of super effective coverage, especially Groudon and Rayquaza can take advantage. Thunder also lacks reliability in sun, and doesn't really hit anything else of note.
Flamethrower hits Metagross and Forretress hard, and gets a sun boost, but it doesn't really hit anything else of note, and both can be dealt with decently with Aura Sphere, whilst Grass types are equally hit hard with Ice Beam
A partner is Thundurus, which can use priority Thunder Wave to paralyze all scarfers that could potentially revenge kill Mewtwo. Also, a Sp.Atk passer like Baton Pass Nasty Plot Mew can be useful, as after Mewtwo receives a single Plot boost, it achieves world domination. Stealth Rock layers also help Mewtwo, as with Stealth rock support it can OHKO so much more (especially Lugia, where it cancels Multiscale).