Oooh shit lets see USA would be a very nice choice. Considering what they've done to the rest of the world total annihilation can repay some of the blood debt. Not to mention I think most of the people on my internet hate list are American so I would happily wipe out 300,000,000 just so they can die. I'd consider England to wipe out the chavs, and the descendants of those bastards responsible for the Empire. Italy to destroy Vatican City, and so Rome will burn. China, just for the largest death toll. And then the others to consider would be France, NZ, Turkey, Israel, Mongolia, Jamaica, Sudan, Greece, Pakistan and India
I wouldn't do anything of the sort. But if I had to pick, the only one that comes to mind as reasonable, would be my own fucked up USA.