i also think the definition of horror is very subjective; it’s a very personal genre, and examining exactly what about the films you’re watching you find scary (and why) is key to understanding how a film works
here are some of my fave horror movies (some of which people debate about whether they’re horror or thriller or whatever, but to me, they are firmly rooted in horror technique and aesthetic): M, the shining, a clockwork orange, carrie, les diaboliques, get out, us, the lighthouse, whatever happened to baby jane?, the hills have eyes, the texas chain saw massacre, les yeux sans visage, the night of the hunter, black swan