I generally agree with these lists, and generally can't add anything more to what true connoissuers have already said.
So I've compiled a list of movies that are not considered "horror" films, that nevertheless scared me in different ways, often profoundly, sometimes in ways I can't explain. I would love to hear if any readers have been terrified and shaken by mainstream films that are not in the horror genre.
Here (in no particular order) are my 6 choices. Have fun!
If Kyle MacLachlan's discovery of a severed ear in a vacant field isn't enough to unsettle you, then what he observes while hiding within Isabella Rossellini's closet should do the trick. Dennis Hopper's Frank Booth is pure, unredeemable evil. David Lynch unleashes an inferno of disturbing images and dangerous emotion, and plumbs the rot beneath seeming innocence. Rarely have I been as startled and uncomfortable, even though I could not force myself to look away.
2. FELLINI SATYRICON
The director's characteristic generosity turns malevolent. He confronts viewers with the stuff of his nightmares, in a world that is as alien and yet ominously familiar as the fall of Rome is to modern culture. It's an eerie procession of images of gluttony, violence, and decadence, while haunted and haunting faces stare back at us. Fellini immerses us in a Nighttown of hermaphrodites and madmen, decapitations and pestilence, nymphomania and cannibalism. Even the sound and music are unworldly. Truly, this is Fellini's science-fiction version of the ancient past.