Clearly GLaDOS. Because (I'm not joking here) she can tempt you with cake. We all thought we were getting cake, if you didn't see the print on the walls you really thought you were getting cake. She'll cloud your minds with cake. Well her or the Lich King. Millions of Undead minions and a fortress it's extremely hard to get up to? Oh yeah and 100-feet-tall Undead Incredible Hulks? You're screwed.