I'm a little surprised that McDonald & Giles hasn't shown up yet as King Crimson has been mentioned several times. Hard to pick a side as it is a complete piece.
Other selections...
Nice/Ars Longa Vita Brevis
Jethro Tull/Thick as a Brick
Moody Blues/A Question of Balance
Nektar/Remember the Future
Emerson, Lake and Palmer/Tarkus
Frank Zappa/Hot Rats