Been watching a lot of old movies, but I don't know any old GOOD horror movies, the only one I've watched so far is The Thing by John Carpenter, and I've been craving more of it.
I suppose it really goes without saying, but I adore the original Halloween. I’ve never see any of the others and never intend to, but that one is a *movie*’s movie.
If you're looking for something that also has a slow pacing like The Thing: The Wicker Man (the overall execution of the film is pretty bonkers despite that).
Now, if you want a film that's also full of practical effects leaning more towards body horror: The Fly.