The Floyd's masterpiece is Shine On Parts 1-9. I'd agree that Bach's masterpiece is the Goldberg Variations. Other ones:

Genesis - Firth Of Fifth
Jethro Tull - Thick As A Brick, Part One
King Crimson - Larks' Tongues In Aspic, Part One
Mike Oldfield - Tubular Bells, Part One
Yes - Close To The Edge
Van Der Graaf Generator - A Plague of Lighthouse Keepers