Best Horror Movie Ever?

I'm curious what other people, especially horror fans, think are the best horror movies of all time. There's some obvious classics that people think of right away, such as A Nightmare on Elm Street or Friday the Thirteenth. Those were scary movies, but I don't rank them at the top. The first movie in each series was pretty original, but even they still relied too much on making the viewer jump.

One of my personal favorites is Blair Witch Project. I would rank that one probably top five all time (I'm estimating: I don't have an official list). I really like the descent into fear the movie shows. At the beginning, the characters are having fun and above the silly story, but by the end they are reduced to little more than the children in the legend. But what I really like is how successful the movie was at letting you know exactly what killed them without ever showing it (the witch). One of the scariest things is the unknown, and a lot of horror movies suffer from the struggle with this concept. They roll along, pretty good and pretty scary, and then they reveal the monster, it's a man in a hair-suit, and the fear is gone. This problem is one reason why I never watch the end of the TV mini-series It. The spider-thingy at the end does not scare me at all (the one in The Return of the King was pretty intense, though). Blair Witch gets around having to show the monster without leaving the viewer too unsatisfied. A lot of people criticize this movie (some for good reason, some because the shaky camera work confused them), but I think it is one a true great among horror movies.

What do I think is the best horror movie? After all that talk about Blair Witch, I would vote for The Exorcist.