Some good choices so far. I always enjoy these threads.

However, I call shenanigans on the whole "the unseen is scarier" theory.

Let's say you are out camping in the woods by yourself. What would be scarier:

1. The sounds of children's laughter outside your tent.

2. Demonic clown bursts in wielding a chainsaw.

Don't get me wrong: I did enjoy The Blair Witch Project quite a bit, but I didn't find it scary.

You obviously don't have children.

Hellraiser 2 gave me nightmares for about 6 months. Very disturbing...

If you have truly have been lost in the wilderness you can relate to why people find The Blair Witch Project scary. At least this is why I find it scary.

To this day I don't like to watch the Exorcist at night. It is just a scary movie no matter how many times I watchedit.
I saw Poltergeist when I was seven and I was freaked out by the
Kubrik's The Shining was always a very unsettling movie. (the old lady and the twins)

I have to second the nomination for "It." I saw that clown in my nightmares and it made me question the dark (and quiet light for that matter) for a while after I saw it. Very effective, albeit for the unscary ending. The exorcist, too, scared me very much and still chills me to this day. I also have to agree with the posts about Exorcist III, which was fantastic and can be had for a bargain to boot.

another vote here for Ju-On: The Grudge. There are a few scenes that I've never seen anybody just sit through, they freak out, mostly due to the sound design. I have the DTS version, and the sound that the "thing" makes truly makes your hair stand on end, and it circles the room constantly. It kind of throws you off balance, it's the strangest effect I've ever seen in a horror film.

Also, "Dark Water" has a truly creepy ending.