Loki. Obviously. Tom Hiddleston plays the character very well – especially considering that he seems like one of the nicest guys in the world in reality. Loki isn’t just a cliche villain; he has backstory, motivation, and some psychological/emotional basis for his actions. In The Avengers, you hear him twist the truth to justify his beliefs and actions. The saddest part of all is that he seems to actually believe his own lies. That makes him a good villain, in my opinion.