Within Europe, probably the English (as much as I hate to say it) and the Germans. "Dislikes" against nationalities within Europe are usually quite jokey, though. People say that the Germans have not sense of humour and the English are pompous, but no one actually wants to kill the English or the Germans.
Worldwide, I guess the Japanese because they're hated by everyone else in the Far East or the Americans, because people are stupid and jump on the anti-American bandwagon.