For me, It's The human centipede. 11 years later, I still think about the goddamn movie way too much every day. The whole plot, atmosphere and images of the movie are, in my honest opinion, the most horrifying thing anyone could ever think of. I've seen a lot of fucked up movies the last decade, including the most popular ones like A Serbian Film, Tusk and Martyrs and other unpopular ones like Trauma and Strange Circus. Yet nothing even comes close to the agony and emotional torture I felt while just LISTENING to what THC was about.
So which is your pick?