From what I've seen...
Korea (By the time you arrive, they're already an era ahead)
Russia (Tend to go wide, large military, and full rationalism)
Rome (Wide and large military, usually get going early and city-spam)
Germany (Conquers and snowballs)
Poland (Pretty obvious here)
Shoshone and Greece are more so civs that are horrible to play against, but they're usually not world powers, at least not as much as the one above. Any AI can get strong in certain situations, I've had plenty of games where the Shoshone and Greece have struggled and were near bottom in lots of categories on InfoAddict.