I can't believe I'm the first to post this set, but Reshiram can easily pull off a
Reshiram @ Life Orb
EVs: 100 Sp Atk, 252 Atk, 156 Spd
This moveset has both a physical and a special version of the classic fire/dragon combo that gets perfect coverage and also happens to be STAB. If the defenses are more or less equal, use Flame Charge to boost Reshiram's speed, making it faster than Mewtwo. Alternately, put all 252 EVs into Spd to outrun normal form Deoxys after a single boost.
If you're creative, don't copy and paste this into your signature (Started by 3.14kachu).