[The Artist Formerly Known as Hawk]

Hellgate.

Nothing more fun than opening up a doorway to a realm of infinite demonic entities in another universe and allowing them to ravage the earth.

Either that or nanites. Order them to replicate, using any and all resources. Eventually the whole planet becomes Grey Goo, then the moon, then all the other planets, eventually it works its way up galactic threat level.

If not either of those then I'd find some way of breaking apart the strong nuclear force with a blast of energy, make it self sustaining so that once it's set off it continues on through space never ending, then set of a Reality Bomb ala Davros and destroy the entire universe.