It depends on your definition but Visitor Q by Takashi Miike would probably offend most people. His other films can be rather out-there too.

Also the splatterpunk films of Yoshihiro Nishimura rank high. If you want a distilled taste, see "Z is for Zetsumetsu" in ABCs of Death. Otherwise, try Tokyo Gore Police.

Tetsuo: The Iron Man actually caused me to have palpitations, and it's rare I have such a physical reaction to a film.

And that's just Japan, you could always try something like Adam Chaplin.

However, they tend to be so over-the-top that they are entertaining. If you want to watch something that's an ordeal to get through then you could start with the August Underground films. There are others in the same vein like Tumbling Doll of Flesh or Guinea Pig: Flowers of Flesh and Blood. You can then grub around looking for similar films as there's always someone out there trying to be more appalling. However, it's not really my cup of tea - the worst I've seen in the cinema is probably Necromantic last Valentine's Day, which got quite a few walk-outs, although my main issue was it seemed far too loud.