For movies, I think The Thing sets the standard. The creature is repulsive, but the movie isn't graphically violent and doesn't use jump scares. It's creepy and horrific, but not disgusting or shocking. That's what I expect of horror movies.

Strangely, I think I saw Alien first at 15 or so and I never thought of it as a horror movie until much later when other people kept mentioning it in horror movie discussions. But then, I practically grew up on Star Trek TNG, which has a couple of episodes with somewhat related structures and concept, so I think I watched it as a more dramatic staging of an already familiar story. That might have taken most of the punch out of it.

One of the greatest things, similar to The Thing, was when I started with manga and wanted to check what I have heard, that there do exist actually scary horror manga. And I have to say, yes, Uzumaki is the proof that it does.
That thing is so damn messed up. That's straight out lovecraftian. And even for Japanese standards it's bizare. A remote fishing village where people suddenly start getting obsessed with spirals and weird natural events and accidents with unexplainable spiral patterns occure. And the one kid that got to go to a university in some big city and is on a visit to his family is the only one who notices that the village had always been strange compared to the outside world.