A Clockwork Orange
The concept is not as sick as A Serbian Film but it depicts a lot of disturbing scenes on psychiatry, juvenile delinquency, youth gangs and other social, political and economic subjects. It's such a brilliant movie if you put aside the uncomfortable repulsiveness of the lead actors. I'm used to that kind of violence since most of the TV shows I watch involves a lot of it, but I'm sure not everyone will be comfortable with it. Stanley Kubrick did such an amazing job with this film, hats off. Oh, and it also reminds me of Panic! At The Disco's I Write Sins Not Tragedies.