I have to agree with you, at least as the most horrifying movie I've ever seen. I actually stood up and started to leave the theatre, I didn't think I could handle it, but I found the story so compelling I sat back down. Sloth is what made me almost leave, so disturbing...
As far as good fun screamer scary movies, I always loved watching the original Halloween and the first sequel, When a Stranger Calls, the first Nightmare on Elm Street, and the original Prom Night. There was also a made-for-TV movie I used to watch as a kid called This House Possessed - I am sure it was terrible, but I loved it.