This one's easy: I'd go with a full raid group into Dragon Soul and kill Deathwing. This isn't something I'll be able to do for a long time, like, for another expansion or two, so if it was my last ever day of playing WoW, I'd take down the biggest boss there's been so far. Deathwing isn't my favourite villain, but he's perhaps the canniest, sneakiest and most intelligent of them all. He had immense patience and amazing planning skills. Unfortunately, none of this came through in Cataclysm, which seriously sucked, but I still count him as the biggest villain we've faced as of yet.