The Exorcist was scary for it's time. Not so much anymore. It didn't hold up well.
It's kind of like the way our kids are going to look at us when we tell them that Michael Jordan was the greatest to ever play the game:
The Shining holds up much better. Watch that shyt in a pitch black room.
So my list is
1. The Shining
2. Alien
3. Original Texas Chainsaw Massacre
4. Original Halloween
5. The Thing
Honorable Mentions
6. Poltergeist (more of a childhood thing)
7. Deliverance
8. Let the Right One In (the original, though the remake is pretty good)
The best time I've ever had at a horror movie though was definitely Sam Raimi's "Drag Me To Hell" or "Freddy vs Jason". Those weren't that scary to me though.