Cruelest of movies?
I'm hard pressed to think of a movie that depicts sheer cruelty in such a way.
Irreversible? More a depiction of rage.
A Serbian Film? Close, perhaps, but the protagonist is so disordered by the end that it's not clear how aware he is.
Antichrist? No. Too much prior psychopathology at play.
Martys? Close, but the motivation behind the savagery redeems ever so slightly.
Human Centipede? Too outre
Last House on the Left? Redeemed slightly by successful revenge.
Men Behind the Sun? Savagery that is ultimately bureaucratic, mechanistic.
I Spit on Your Grave? Same as Last House...
Funny Games? Again, close, but too obviously experimental, constantly reminding the viewer of the filmmaker's intent.
Salo? Not really a film of cruelty as much as an orgy of sexual extremes.
Audition? Again, ameliorated by vengeance.
The Girl Next Door? Again, close, but not quite.