Here's sum mo "shiznat":
Arnold Schoenberg ("Pierrot Lunaire", "A Survivor from Warsaw"). True experiences, very visual and evocative music, that you don't even notice how highly organized and complex it is. Some of the best musical "journeys" ever.
Giacinto Scelsi ("Quattro Pezzi per Orchestra"). Ghostly music, built around permutations of a single note (!), but very rich and yielding nonetheless.
Karlheinz Stockhausen ("Gesang der Junglinge"). The originator and mastermind of "troo" electronic music, this is a piece built of manipulated and generated sounds to present a real drama and a work of universal scope.
Anton Webern ("Symphonie", "Variations", "Passacaglia for Orchestra"). I must mention him because his music is so far the hardest for me to "crack". It is quite "nice" and "melodic", and the pieces are rather short and compressed, but so far I can only grasp bits and parts of patterns inside. I'll keep on listening. If somebody "gets him", my sincerest congrats.
Igor Stravinsky ("Petrushka", "Rite of Spring"). The composer takes a simple and memorable folkish motif, introduces it, and then deconstructs it to smithereens, until he gets to the "primitive" but very intricate rhythmic sections that make all "brootal" death metal seem puny in comparison. Other composers of this kind are Sergei Prokofiev (the piano sonatas) and Bela Bartok ("Allegro Barbaro" and "Mikrokosmos").
Yesterday I went to this cool concert in a small church just around the corner from where I live... modern vocal+piano music, they played Schoenberg and simply Berg and several others...wow.
I've yet to see the end of it. Lots of stuff to discover!
regardz,
D Mullholand