Two come up to my mind immediately, the rest of the names you'll find on web have some good parts but not as extreme as these two.
1) ichi the killer , you can find the uncut full version on youtube
type ichi the killer uncut part 1
2) Salò, or the 120 Days of Sodom (if you can get your hands on it)
Don't watch these if you haven't got strong stomach.