A Serbian Film was kind of meh for me. I expected more. Audition, I didn't get the hype behind. It was a decent film, but wasn't disturbing to me.
Check out Salo, or the 120 Days of Sodom. Most would find it fucked up, I've just desensitized myself to damn near anything.
Have you watched the other August Underground films? You could keep the trend going. Maybe check out some David Lynch classics like Eraserhead. Irreversible I've heard good things about, though I haven't gotten around to watching it personally.
Also, don't bother with either of the Human Centipede films. They are overhyped and don't nearly deliver.