Tough one, but this is what I've come up with. I guess some predictable picks, but they're popular for a reason. (Limited to one album per band, and ordered by year of release, because I find it difficult to rank favourite music from best to least best.)

King Crimson - In the Court of the Crimson King
Caravan - In the Land of Grey and Pink
Jethro Tull - Thick as a Brick
Gentle Giant - Three Friends
Genesis - Foxtrot
Pink Floyd - The Dark Side of the Moon
Van der Graaf Generator - Godbluff
Camel - Moonmadness
Rush - Hemispheres
King Gizzard & the Lizard Wizard - Polygondwanaland