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.