Granted, I don't usually get to see many movies in the theater, so I'm not saying there are a whole lot of competitors for the title. I did see Blair Witch in the theater, but I took so long to figure out what was going on in the two freakiest parts (What's that in the shirt? Why is he in the corner? OHHHHHHH) that it was more of a delayed-reaction terror.
So. The Ring. I went to see it by myself because no one would go with me, but I was doing movie linkspams on a dinky GeoCities site (yes, even then), and I thought the trailers looked really cool and creepy and I had a feeling it was going to be a really big thing. (And I still think that the trailers and the Cursed Videotape, copies of which the marketing team actually planted in real life, were amazingly well done--just the right touch of icy, cryptic distance.) So I went to an afternoon showing, probably a Sunday, and I was pretty much alone with my oatmeal fleece jacket and a small clutch of college students (my age) a couple of rows behind me. I nearly died. Naomi Watts fell into the dark haunted well and I wanted to leave the theater so badly, I have never wanted to leave a theater from terror before, but my legs no longer worked. I am serious, I sat there, literally, in the literal sense of literally, trying to get out of my seat and I could not make it happen. I then began to pray, in complete seriousness, that a higher power would strike me unconscious before Naomi Watts found something. Because you knew she was going to. There was no point in the movie booting her down there otherwise. You knew. And the movie. still. made you. wait for it.
(I can't find the entire scene on YouTube, but here it is right up to the part where It Got Worse, as the Tropers would say. I think the end is where I started to lose control of my motor capabilities.)
When it was over, I staggered out of the theater white as a sheet. I am not exaggerating a whole lot on this one. "What happened?" my mother asked. "You look like you've been through a war."
I never saw any of the other movies, Japanese or sequels or prequels or whatever-have-you. This is all I want to have seen.
Obviously, this is your cue to share a movie that has scared you stupid. I want experiences of real, actual, physical terror if you've got it.
NOTE: ALSO: TCM is running The Innocents later tonight; don't miss that if you found the discussion interesting.
ETA: I have been reminded of Tiny Moist Hand!