If you're looking at most violent in terms of quantity, i'd say
Hard Boiled stands out.
If you're talking most violent in terms of pushing to extreme i'd say
Audition (Miike, again
); it's violence that really gets under your skin more so than the comic book glossy stuff in
Ichi.
Gory violence i'd say
Bad Taste and
Braindead take the biscuit.
Kung fu violence, can't not mention Lee in
Enter the Dragon but i'd put
Ong-Bak over that.
Oldboy has been mentioned but i'd say except for the odd (brilliant) scenes of fighting, it's mostly torture that counts towards violence, so if you like that
Save the Green Planet is worth a look, and if you like the fighting violence of it definitely check out
A Bittersweet Life.
For plain whack violence-
Assault on Precinct 13 and shooting the kid in cold blood,
The Texas Chain Saw Massacre for the unrelenting nature of violence directed to Marilyn Burns, also
Urotsukidoji for crazy monster rape.
On subject of sexual violence there's
A Clockwork Orange but
Irreversible is waaay up there and i'd almost put it as my answer.
I'm sure i'm forgetting some gems of sado-masochistic delight and such but can't remember them, got a film at the back of my head that has RAW violence, not glorified or cinematic violence but actually difficult to watch, though it's eluding me.