Try the French flick "martyrs" Not as sick as Serbian, but quite shocking in itself.
Hostel, clock work orange, human centipede, I spit on your grave (OG), murder set pieces, funny games, oldboy, high tension, I stand alone, begotten, ichi the killer, inside, August underground, irreversible, Audition, cannibal holocaust, necromantik...
IMO Serbian is the most shocking film I have ever seen, followed by Martyrs...
If I were to watch the most fucked up movies ever, Id watch those 2 last..