Best horror film ever? I'd say that's between The Shining and the classic Exorcist. Both of those (along with Hitchcock's Psycho) completely changed that movie genre.

Honorable mention goes to Rosemary's Baby, another classic.

Does Aliens count? Cuz it would certainly be my favorite if it does. I really can't stand horror movies, with their cliches and brainless characters and impossibly stupid situations. The modern horror film is the worst, relying on improved makeup and special effects to scare people with more gore while failing to improve on the time honored "spooky music" "startles" and other cores of the genre.

Candyman scared the freaking shit out of me when I was a little kid. It made me afraid of the dark again... But other than that, all semi-recent horror films have failed to impress me.