I'm a huge horror movie enthusiast but they have to be a lot better than all the cliche'd "cat jumps out of trash can", "buddy's hand slaps on shoulder" loud-noise + jump-scare crap that you see in most movies nowadays. It has to have a plot, good story, characters you can identify with, and suspense...something you rarely see with modern horror movies.
For movies that were scary back in the day, the first 2 Halloweens, The Thing and The Fog (I'm a big John Carpenter fan), Carrie, The Shining, Evil Dead, Jaws, The Amityville Horror, Invasion of the Body Snatchers, Alien (I think this was the first horror movie my brother and I were able to go to with our father due to the age restrictions), It's Alive (the commercial for this one, with the camera slowly panning around the crib with creepy music until you see the clawed hand hanging over the edge was particularly scary for me, though the movie itself was a bit less scary)...and Trilogy of Terror, the one story with the creepy pygmy headhunter doll that comes to life and tries to kill the woman who bought it.
For modern horrors definitely worth watching, check out Mama, Midsommar, The Babadook.