Texas Chainsaw Massacre (1974): This should be the scariest movie of all time. The girl running from a big guy with a chainsaw and skin on his face.
The Silence of the Lambs: The Silence of the Lambs is easily hands down the best horror movie I've ever watched. The movie itself doesn't throw countless buckets of gore at you
The Thing: This movie needs to be higher. It is a classic horror film with great characters, settings, suspense, and gore, and you won't be able to forget it.
Scream: This movie totally gave the slasher genre (and really all horror) new life and started a new trend of smart suspense-driven horror movies. The entire franchise is classic.