There are a few I left off because I didn't really consider them true horror movies. Army of Darkness (too much of a slapstick comedy), Jaws (more of an adventure/thriller), Aliens (sci-fi/action as opposed to the original's sci-fi/horror), Silence of the Lambs (a straight-up crime thriller), and The Wicker Man (also a thriller) being the most notable. Anything else "missing" probably isn't. I just wouldn't rate it as highly as some would (i.e. Halloween, Night of the Living Dead, Friday the 13th, etc.).