My vote has to go to Alien, the original film. Great cast! Strong female character (also a black actor who survives for a little while) which is unusual in traditional horror or spect. Good plot twist (the "company" wants to use the monster as a weapon! Don't tell McCain!!). Also, like any haunted house (spaceship or whatever), there were plenty of dark creepy places for our luckless characters to tiptoe through. Don't open the door!!!
There are two that still give me chills when I think of them, Candyman and of course, the Exorcist. Scarey stuff. Nothing has really been as frightening for me as these two films.
Candyman introduced a African-American spiritual monster. He was unmerciful, but quite peaceful while being so.Lol While I had seen Blacula, I was not impressed by the film in the least. The male character in Candyman thrilled me and I wanted to see more characters like that.
And the Exorcist was just plain terrifying. The book was even more so. The Exorcist inspired me to want to write horror. I wanted to create a story that was frightening, but that also held a moral message.