The Thing is my favorite; great combination of suspense and horror. It holds my attention throughout and has a great ending.
The Shining kind of elevated horror with a more artistic take and is a classic haunted place story. Great performances too.
Halloween is the greatest slasher film and really got the genre growing spawning tons of copycats. Michael Myers is an iconic character.
It’s gotten tough to narrow it down now, leaving out classic and influential movies like Alien (maybe too sci-fi), The Texas Chain Saw Massacre, Poltergeist, and Psycho doesn’t feel great.