It’s tough to say - I tend to consider books that have stayed with me the ones that scared me the most. Off the top of my head:
- The Last Days of Jack Sparks by Jason Aarnop
- From A Buick 8 by King (though I’d say I found this one more existentially disturbing and unsettling)
- Drood by Dan Simmons
I think, looking at this not-at-all-exhaustive list, that the books I find scariest are the ones where you’re left with more questions than answers.