I think The Exorcist is hands-down the scariest movie I've ever seen in my life. ALL movies related to demonic possession and the Anti-Christ freak me out, but this one tops them all by far. It's also in the Library of Congress's National Film Registry, been named the "scariest movie of all time" by several different media outlets, and is one of the highest-earning movies of all time (source: Wikipedia).
As I've said before, I've pretty much always been allowed to watch horror films. Most of them never scared me beyond when I was actually watching the movie (and some didn't even scare me then, but I guess that's beside the point). But this one? Oh, my. I wasn't allowed to watch it for the longest time, and I think that was because my aunt had seen it in the theater (or the drive-in, I'm not sure which) when it was released and it really, really, REALLY scared and disturbed her. In fact, I've only ever seen the version shown on tv - and not even cable tv, but one of the "Big Three" television networks.
So I finally convinced my parents to let me watch it when I was about 12, and they sat right there beside me, and said that I MUST cover my eyes, or possibly even shut my eyes and stick my fingers in my ears whenever they told me to. Of course you all know I cheated, and watched all the graphic scenes through my eyelashes and didn't do a good job of plugging the sounds out of my ears, either. And it was hard sitting there trying to act like I wasn't seeing and hearing everything (at least the sounds, I probably couldn't tell you the actual words even to this day), when I had
I've since come to learn that the novel The Exorcist (upon which the film is based) was merely "inspired" by actual events. That's a world of difference, especially to an adult as opposed to a child. But things of this nature will always both intrigue and frighten me, as I truly believe there is so much out there in terms of good and evil spirits, energies, etc, and there are a lot of things that more practical people would dismiss that I just don't find that farfetched.
Image Source: Wikipedia