Pretty much the Nathrezim as a whole. They've done more damage to Azeroth than any other part of the Burning Legion could've ever dreamt of achieving, if you factor all the shit in which they triggered. Azshara/Medivh/Archimonde/Kil'Jaeden's Horde + Scourge didn't have half the impact the Scourge under Arthas' leadership had (that might be a bit over exaggerated, but you get my point). Also, Archimonde wouldn't have even been able to enter Azeroth without the Dread Lords.
EDIT: Forget my first point, I forgot about the whole War of the Ancients thing...