I go for the ones that unsettle me long after I've closed the book. There's a line from 'The Haunting of Hill House' that does it for me: 'Whatever walked there, walked alone.' It's not loud or gory, but it makes the whole house feel so empty and wrong. It's a great caption for a moody shot of a dark hallway or an old, empty staircase.
For something more direct, Shirley Jackson's other classic gets under the skin. 'No live organism can continue for long to exist sanely under conditions of absolute reality.' That first line from 'We Have Always Lived in the Castle' sets a tone that's both clinical and deeply unhinged. It pairs well with slightly surreal or decaying imagery.
Honestly, modern horror often nails it too. Paul Tremblay's 'A Head Full of Ghosts' ends on a note that's just... quiet dread. 'The only monster here is the one I created.' That one hits different if you've read the book, but even out of context, it suggests a horror that comes from within, which is often the scariest kind.