I saw someone mentioned Exorcist III, and I'll echo that. While not as great as the first, it is far above the second one.

I've got a soft spot in my heart for:
Hills Have Eyes (Remake. It had a ton of gore, and the story about a father searching for his kidnapped infant was great.)
Evil Dead (most people talk about Evil Dead 2, but I still love the first one. It was more of a straight horror film then Dead by Dawn, which had some goofy moments, but I still like it a lot. I'm not as partial to Army of Darkness because I felt that one was too goofy.)
Silent Night, Deadly Night (1984 version. I watched it with some buddies in High School, and we had a blast.)