Hmmmm.

There are so many horror movies that I love, it's really hard for me to choose! Many of them aren't so much 'Horror' as they are 'MindFuck' movies. Thinks hard.

Yeah - ok. I'll have to go with Psycho. Hitchcock really was the ultimate master, you know?

Other contenders would be The Birds, The Shining, Cube, and Phantasm.