I'd say The Exorcist is still my "scariest movie of all time". The Sixth Sense got to me, too.
I used to love horror movies, books, everything, but now I'm a total wuss and can't watch anything. A little bit ago, they had a "creepy gif" thread on CEP when I was lurking, and at the second one I freaked out and was like, close window, close window, close window!