The only time I came close to walking out of a movie theater because of unbearable tension and, yes, fear, was when my husband and I saw Wolfen, early '80s. He asked me if I wanted to leave, and I almost said yes, but decided to stick it out. I guess there was something going on in the theater that night, some mass fear or something, because no one else even seems to remember that movie, yet it got to me as no other ever has, before or since.
It was really, really creepy, the way they used the city-scapes of the derelict neighborhoods in NYC (which I had never even seen before, hadn't even known they were there). Then, too, you saw much of the action through the eyes of the supernatural predator, and that was creepy, too. For a long time you didn't even know what it was or why it was around.
I have The Descent on DVD. It is a good horror movie, but I have trouble with it for two reasons--can't stand the death of the child early on, and I dislike that no one escapes.