There’s already a thread about new horror movies so I thought I would look for some recommendations about old movies as well.
My personal favourite is the black and white 1957 horror movie Night of the Demon. It’s a brilliant psychological horror about the conflict between the rational scientific worldview, represented by the sceptical (American) psychologist Professor Holden, and the magical occult worldview represented by the simultaneously amiable and sinister (British) witch cult leader Dr Karswell. The special effects are actually pretty good for an old movie, but it’s not a film that relies on special effects to convey its sense of dread and impending doom. Kate Bush sampled the seance scene for the Hounds of Love so guess she’s also a fan. I think I’ve watched this film even more times that Ferris Bueller’s Day Off !
Also worth mentioning here is John Carpenter’s In the Mouth of Madness. Admittedly it’s not as good a movie as They Live or The Thing, but as John Carpenter’s nod to Lovecraft and the mythos it’s definitely worth watching.