Anything prog and metal (although my tastes in metal are a little bit more particular). Some good bands include: Opeth, Dream Theater, Transatlantic, Symphony X, System of a Down, Metallica, Anthrax, Rush, Pink Floyd, King Crimson, Yes, Genesis, The Who (more "classic hard rock/blues rock" than anything, but still love 'em), The Beatles (some of their more progressive stuff from Revolver to the White Album), Led Zeppelin (what fan of hard rock doesn't like them?), Black Sabbath, Avenged Sevenfold, Ayreon (epic prog metal opera cycle by Dutch composer Arjen Lucassen), Devin Townsend, Azazello (great Russian prog metal band), Mechanical Poet (another great Russian prog/post-prog metal band), Queensryche, Kansas, Uriah Heep, and the list goes on. I also enjoy classical quite a bit - composers like: Beethoven, Ginastera, Chopin, Schoenberg, Webern, and some film composers like John Williams and Howard Shore.
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"Works of art make rules; rules do not make works of art."
-- Claude Debussy