Chalk up one more vote for "The Exorcist". I've seen it several times and every time it creeps me out. It also has the power to leave me thinking afterwards. Something most horror movies don't do. It also strikes me that the movie was based on actual events.

The movie "The Shining" also ranks high on my list. It was the first horror movie I ever saw and still gets me out of my chair at times. Heeeeeerrrrrrrrrrre's Johnny!!!