I love horror but apparently I don't vibe with a lot of recommendations I find online.
I'd seen so much hype about Event Horizon and I absolutely hated it and didn't find it scary. I just watched Late Night With the Devil and whole it was definitely enjoyable, it wasn't the least but frightening. I also just watched Let the Right One In and really didn't like it. It also was not a horror movie in my opinion.
I will say one of my favorites is Sinister or the first Conjuring. Sinister for the stomach twisting dread and suspence throughout and Conjuring for the same.
I know it scared the shit out of me the first time I saw it.
I was 15 and my family rented it. So I didn't get to see it in its full theater screen glory. Just a 25" console TV from a well worn VHS cassette where the top 25% of the screen was wavy.
I think Alien is a great movie (and definitely horror), but whether it scares modern audiences is pretty hit and miss. It's very slow paced and while I love the practical effects, the alien looks downright goofy in some scenes. I certainly don't find it scary having seen it, and new viewers I've shown it to usually aren't that scared unless they're self-identified wimps when it comes to horror. Aliens is scarier I think, even though it's more action than pure horror.
Same goes 10x for The Exorcist. It tops a lot of "scariest movie ever" lists online, but watching it today is more comical than anything. I think you have to be scared of demonic possession actually happening IRL to get scared by that movie.