Gotta say... The Exorcist.
I saw this when it first came out, in the best of circumstances. I was 18, my sister was 16. She managed to get in to see it with her boyfriend (who was 18 and a good friend of mine...until he started to date my sister, but I digress). They came home about an hour nad a half after leaving saying they couldn't sit through the whole movie. My sister had nightmares for a week! All her boyfriend could talk about was "the CROSS...Oh my GOD THE CROSS."
I HAD to see it. Then about week later, we had a snow day from school. By afternoon the roads were clear and my mother, of all people, says, "Let's go to the movies. You drive, I'll buy." She didn't have a license. So of course I'm in. We go to one of those old style theaters. One screen, balcony, big red velvet curtain. We go in and the place is packed, and just buzzing. Then the lights go down, and the curtain opens to that black screen, the creepy theme starts and then the big red letters... "the Exorcist." No commercials, no comming attractions, nothing. Just the movie starting. I clearly remember the anticipation, thinking, "come on baby, SCARE me!" I was not disappointed.
I thought the worst of it was when they were doing all of that stuff to her in the hospital. Sticking the thing in her neck and the blood spurting out. MAN! That stuff really happens.
Anyway, a year later I'm at college in a little town and the Exorcist FINALLY comes to town. All these kids that hadn't seen it wanted to go. So we go and they're showing comming attractions of BLAZING SADDLES. Talk about killing the mood.
Then, a few years after that, I'm teaching at a private boarding school, and I have to live in a dorm with a bunch a 14/15/16 yr old boys. The Exorcist was comming on TV and they all wanted to watch, but they weren't allowed TVs in their rooms, so I said, OK. BUT...all homework has to be done beforehand, once it starts nobody leaves until commercial break, and no screwing around. I started out with about 10 kids, and in an hour was down to two!
I gotta go with this one.