Can you describe what is scary to you?
A world without beer, that would be scary!
Sorry, got side-tracked. What do I find scary film wise? I mentioned the films that would be in my top five. As I said, these things are subjective. But I will elaborate on my point of view for the six films I listed:
Jaws- Because this was inspired by a true story (Jersey Shore attacks at Matewan Creek in 1916) And it's not like sharks don't exist and attack people. It happens. This film works so well because you don't see the shark for the majority of the film. Just like in a real-life attack. You wouldn't see Mr. White until he attacks from below. Good-bye leg! Crunch! Crunch!
Alien- The chest burster scene alone puts this in the top five. Look at the faces of the people in that movie when it happens. The only people who knew what was going to happen in that scene were the guy playing Cain and Ridley Scott (the director). Those screams and looks of horror are the real deal. Plus the HR Gieger Alien is frightening.
The Thing- the best modern horror movie IMHO. Why? Isolation. Can't trust anyone. Nothing is as it seems. Freaky monster dogs. Alien that can assimilate whatever it touches. The list goes on.....
The Exorcist- Religious beliefs and overtones aside, this film and novel are basically about the corruption of the innocent. What is more frightening than evil personified in a sweet little girl?
Halloween- The blue-print for the 80's slasher movie. Minimalist approach to the violence, excellent score, doesn't try to over explain the reasons behind Michael Myers actions.
Night of The Living Dead- It's almost documentary like or a found footage type of film. The zombies aren't even the scariest part. The people are. And the stills during the end credits? Chilling.
This is just off the top of my head. And just my opinion