If it's "disgusting" as in "never should have been made" creepy, then my top 3 are:
Bloodsucking Freaks
I Spit on Your Grave
Last House on the Left
I'm ashamed that I ever watched them.
As far as "mainstream" movies I concur with Rondaddy1:
The Exorcist
The Shining
and would add...
Carrie