So much goes into a good fright flick. To me, it's all about the suggestion. Your own mind can always create a scarier monster than anything the special effects department can dream up with latex. Movies that create a sense of true dread, that set up a scenario where you can feel the fear without being overwhelmed by monsters and gore, will win me every time.
I feel that one of the most overlooked elements of a great horror movie is the dialogue. Dramas and comedies are expected to have eloquent or snappy dialogue, but horror-movie lines often get lost in the avalanche of fake blood. Yet a great horror-movie line can chill you to the bone and stay with you for months. It can also break your heart.
Here are my favorite scary movie lines, from Aliens:
Newt: My mommy always said there were no monsters - no real ones - but there are.
Ripley: Yes, there are, aren't there?
Newt: Why do they tell little kids that?
Ripley: Most of the time it's true.
Still creeps me out, even twenty years later.