I can remember thinking that Texas Chainsaw Massacre was extreme when I first saw it in a social setting when it was a banned video-nasty. People were covering their eyes and leaving the room to be ill. These days, it is regarded as a masterpiece of suggestion, compared to the explicitly gory stuff in the multiplexes.
I snook in to see X-films as a lad. Straw Dogs, The Devils, The Godfather and The Exorcist were going the rounds and regarded as strong meat. The unreality of reeling out into a Southport afternoon, having seen Ollie Reed roasted at the stake was a lesson in transgression. But I knew it was a transgression.
We've become rather inured, I suppose. Last Christmas I dozed through Hostel and Wolf Creek - rather cuts off the same old sausage, iirc. In a local video outlet recently I overheard two tiny girls debating which of the Saw movies was the worst. They were about six years old!
Nobody who winces at cruelty should be reading this thread but I'll give a fair warning that I'm about to discuss cruelty to animals.
Hollywood wasn't too fussy about cruelty in its golden years and many Westerns especially are now habitually trimmed by the bbfc for cruel horse-falls. Now and then uncut versions of these movies have been issued as the bbfc have progressively tightened their interpretation of an act passed back in the nineteen thirties. So lots of DVDs on the street, particularly at the cheaper end of the market have been manufactured and distributed uncut, before the bbfc insisted on cuts. One famous shot of a horse's suicidal leap from a high rock into a river turns up in at least two of the John Wayne Lone Star westerns and probably umpteen different movies as well.
Speaking of horse-falls, the bbfc insist on cutting Tarkovsky's André Rublev to remove a shot of a horse falling from steps to its death. The shot was for real and the horse was purchased as cannon-fodder to destroy on screen. Plainly it is terrified. How do I know? Well the bbfc's remit does not cover television, which has screened the film without that cut. The BBC did, however, trim the film to remove a scene in which a cow is set on fire. Again, this was not a special effect.
I like to think that the scene in which a man is trussed up like a mummy and forced to drink molten brass was a special effect but I have my doubts. :?
As for the most extreme film I've ever seen. No question about it: the racoon-skinning in a Chinese market video, which can be found online. Proof, I think, that there is no God.