Which Horror film is the Scariest Ever?

I thought a clockwork orange was not necessarily a horror, but definately something worth thinking about. It freaked me out when I saw it again. I first saw it when I was younger and thought it was alright but remembered that vivid rape scene. Then I saw it again with my husband and we had to shut it off!

A Clockwork Orange a horror movie? Violent and frightening because of the outlook on the future, but a horror?

Or....Session 9 (the haunted asylum movie that only a handful of people have seen), or House on Haunted Hill (the remake). The latter was on cable last night, so they had to edit a few parts, and it was *still* scary. That's the mark of a truly terrifying film. I've watched it six times, and still haven't seen the whole thing.

Gah. Good thing it's daylight out.