I haven't read all the threads yet, so I might have missed this one. The Chorus Line was one of the most frightening movies I'd ever seen, especially when Cary the dancer says, just when you think the movie is mercifully coming to an end, "Can I say one more thing....?"
Other favorites - The Last Man On Earth scared me so badly I had to leave the theater (at 10). Alien and The Exorcist, of course. The Legend of Hell House was particularly atmospheric, and Roddy McDowell was scary as all git out. I still like to watch that near Halloween.
But my all-time favorite - not because it's scary, just my favorite - is Theater of Blood. You get to see Vincent Price and Diana Rigg (yum!) do Shakespeare and some of the most hysterically funny murders on screen. Totally over the top, and it looked like a blast for the actors to make. Robert Morley being force-fed poodle stew through a funnel...whoa...