OOIOO challenging? Not so much. Green and Gold is my favorite, but I love self-titled too. I used to have a boredoms review site..
http://www.geocities.com/boreviewx ...
I neglected from choosing stuff like Mr. Bungle because that stuff's kind of obvious.
I'm listening to Harry Partch - Delusions of the Fury as we speak. Amazing homemade instrument beautiful complex compositions.