I do consider myself to be very big into the avant garde (As much as I hate that word) "scene" (I hate that word too). John Cage is fabulous, and if we're talking about modern day avant-composers, we cannot, in any concievable way, not mention Edgar Varese. It's simply required to mention him because he's simply amazing. Of course, he died in 1965, but I'm going to count him as modern. And he was the major inspiration and role model of Zappa, who's simply a genius. (Zappa wrote a big thing about him, which you can read Here.
It's great to see another Zappa fan.
I like Avant Garde stuff, I suppose. I love John Cage's works-I actually think he's much easier to get in to than a lot of other composers. Varese is, of course sublime, and Zappa is SUPREME.
I've heard some Boulez-it's really fascinating. The main argument against modern composition/avant garde composition is that it's breaking rules for the sake of breaking rules. I disagree-a lot of the pieces follow the rules, simply not how you expect them too.
Honestly, It's absolutley fantastic.