I'm 46, and I had plenty of time in my life to se every horror film that cursed my eyes. And, without a doubt, The Exorcist, has been the only one I have seen, that has left a life changing impact on my personality. I could not even blink my eyes while watching it. And, literally for weeks, I could NOT get a good night's sleep. If you feel you could handle it, get the uncensored version.
Hand's down, the Exorcist can scare the livin' crap out of you for one reason. All the other monsters are imaginary--you'll never see a zombie or vampire or werewolf in real life. On the other hand, being possessed by a demon is a documented malady that NO ONE wants visited upon themselves. Scarey as heck watching Megan go through that.
Back in 1973 I had only been married a year. We went to see the movie and there were people passing out tracks about being a Christian (which I already was). I have never in my life been so scared at that movie. I think our thought process was different back then because I had never seen anything so scary. I slept with the lights on my weeks, went to church ever chance I got and wore a big cross around my neck....OMG....I have never watched it again and I am almost 57. ha ha