Some suggestions from my side - in addition to many already posted suggestions I would add to my list as well:

• The series Twin Peaks (all three seasons plus the feature 'Fire Walk With Me') is not 100 % horror but it has it's tense of mystery and some scary scenes as well

• Don't Look Now from 1974 - a classic mystery drama with great Donald Sutherland in the leading role - also more mystery/suspense than horror

• IT (reboot) - still scarier than the Terrifier movies which are more slasher / fun-splatter

• Quarantine - until I saw the original 'REC'

• Schock - Italian horror movie classic

• Barbarian - I expected different but I was surprised in a positive way

All films from Robert Eggers with his debut "The VVitch" and Ari Aster with its debut "Hereditary" are highly recommended although not all films are 100 % horror movies.