Some suggestions from my side - in addition to many already posted suggestions I would add to my list as well:
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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
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Don't Look Now from 1974 - a classic mystery drama with great Donald Sutherland in the leading role - also more mystery/suspense than horror
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IT (reboot) - still scarier than the Terrifier movies which are more slasher / fun-splatter
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Quarantine - until I saw the original 'REC'
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Schock - Italian horror movie classic
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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.