I'm surprised that Merzbow isn't listen in every post. 'In the Aeroplane over the Sea'? Come on. A decent album, but not remotely inaccessible.
Obviously Merzbow is way up there, I also found Wormphlegm's "In an Excruciating Way..." To be really, really hard to take. Really slow, really evil funeral doom. Khanate's Things Viral still confounds me to this day, and Neurosis's 'Enemy of The Sun' was way to heavy for me when I first got it and it's still a huge slab of (awesome) noise.
I haven't heard much 12 tone stuff but it sounds pretty hard to get (perhaps there is nothing to get?) so I'll put it up there.
Basically, a 3 way tie between Wormphlegm's debut EP, Schoenberg's 12-tone stuff and Merzbow's crazier albums. I imagine there are some albums out there that are nothing but really trebly blasts of arrhythmical white noise for hours on end, though.