On diff levels Regent and below, I'd have to say Russia is most annoying, with England a close second. Russia is the most thuggish at those levels, the only civ that would demand some sort of tribute or use sneak attacks in nearly every game I played with them. On Monarch (highest I've gotten so far), I'd have to go with the Zulus ... they run those damned Impi thorugh your territory, you ask them to leave, they say okay okay, but then keep right on going, finally you say leave and or declare war (why is that the only option at that point?) and _every_ time they will declare war. Not a big deal, because hey, they're just Impi, but those Impi have a 2 movement rate and will zip through your territory destroing improvements + hitting strategic resources. Very, very annoying.