Oldboy:
Oldboy begins with 
the protagonist being imprisoned, in what appears to be a cheap hotel room, for 15 years. He is then released and attempts to uncover why he was incarcerated. At this point you may be wondering what taboo themes this film has to offer; all that I am willing to say is wait until the end.
Cannibal Holocaust
You know a film is at least going to be interesting when a court case was brought against the director accusing him of murdering the cast as part of the making of the film. One of the first found footage films, it tells the story of a documentary crew who had gone into the Amazon forest to film cannibal tribes. Famed not just for its graphicdepictions of cannibalism, it also contains real footage of live animals being butchered.
Tetsuo The Iornman:
Donning a post-modern hat, this film can be described as having a non-linear plot structure that contains themes of rape, repression, violence and vengeance. It is stated by some critics to be a critique of the social and sexual repression in Japanese society. It can also be described as a film where the main protagonist’s penis turns into a gigantic drill while he is having sex, causing him to skewer his unfortunate wife. It’s quite a weird film.
Audition:
This is a film which is about a Japanese businessman who sets up a fake film audition to find a new girlfriend. At first this seems to be nothing more than a saccharine rom-com with a somewhat creepy premise, until the 30 minute mark is reached and the film begins a horrifying U-turn. Containing themes ranging from domination and the lust for power to child rape, it’s a film that does not think much of showing taboo material.
A Serbian Film:
A 
Serbian Film has nearly every theme and image on this list, save for animal cruelty and drill penises. It charts a down-on-his luck porn star who agrees to take part in an ‘art film’ directed by an rich ‘admirer’ of his. Understood by director Srdjan Spasojevic as a portrayal of Serbian society, it is unrelentingly grim and visceral from start to finish; at around the midpoint it contains what is probably one of the most disturbing scenes in cinema. Whether you consider it tastelessly and needlessly offensive, or groundbreaking in its relentless quest to overstep the mark, it’s safe to say boundaries are ignored