It's gotta be Faces of Death Vols. 1-4 for me. I could'nt sleep the first time I first saw it, because I knew that all (well almost) death scenes were from actual video footages, especially the execution by electrocution of one death convict where the camera was even focused on the face of the criminal while it was oozing with blood during the electrocution process.
Also the movie Hannibal starring Anthony Hopkins, the brainy part ending shocked me and made me cringe, since, I have not read the boook at all, and have absolutely no idea that that was gonna happen.
But the ultimate harcore film for me, would be the colored version of the "Unknown Russian Soldier" video clip with complete decaptiation and with DVD clear quality which I downloaded from ogrish.com. I have yet to see something harcore real that wpuld top that.