I dont know how others are interpreting the word "Disgusting" , but for me The Irreversible stands out to be most disgusting movie that I ever saw

After watching the movie the words that come to one's mind are "HOMO-PHOBIC" or probably "MISANTHROPIST"

The primary reason for the disgust are the 2 scenes which have the potential to disturb any Person with a conscience .

The first one is the Brutal Inhumane 9 minute long Rape scene of Monica Belluci

That is true "whole freaking 9 minutes long"...!!! continuous and at a single camera angel , this was the " most walked out scene" of that year , this seen makes you feel as if you have watched an actual rape, it is slow torturous , long and full of anguish .
What makes this scene even more Homophobic is that the Raper in the scene later turns out to be a GAY , making the Rape just an act to prove his might and Disgusting nature. And the violent beating up of the victim after it is even more disturbing. To add to it , the act was not done as an act of Vengeance or a pre-held Grudge it was just done without any cause and out of the probable mental sickness of the Guy.

The Second one is the Brutal murder by fire extinguisher

In this scene ,in an attempt to seek Vengeance , our guy kills this man he founds at the Bar ( who later turned out to be the wrong guy) brutally with a fire extinguisher. He repeatedly hits the man with it , and keeps on hitting way after his head has been smashed into a grotesque mass of blood and bone. What makes this scene more violent is that since the movie is in reverse order the causes of the attack are still not very clear, and it seems a bit excess to do something like that to a guy ( well ofcourse the Rape scene was yet to come , so you might think that you cannot be disgusted any further)

The above two scenes makes the IRREVERSIBLE "The most" Disgusting movie Ever