As someone else mentioned, I think it has a LOT to do with what age you were when you saw it.
I get the impression that a lot of people saw the Exorcist (or movies like Poltergeist) when they were young, and it scared the fuck out of them. Other movies they saw later when they were older might actually BE "scarier" but they didn't affect you as much. I think my judgment is definitely affected by that. My parents took me to see Alien when I was 9 or 10, and it scared the living daylights out of me.
I saw movies like the Exorcist, Poltergeist and the Thing when I was older, and I think they are funny movies, not scary at all.
- same with stuff like Jason, Nightmare on Elm Street, and all that kind of stuff.
I do remember seeing a movie called Xtro in the mid 80s, and I remember that being pretty scary.