So many greats listed already, so I'll add the following: (reasons in brackets)
Yes, The Yes Album (Harder then Fragile, More coherent than Close to the Edge)
Rush, Fly By Night (the most "complete and balanced" rock album ever?)
Radiohead, OK Computer (programmed? overwrought? Who cares!)
Black Sabbath, Paranoid & Heaven and Hell (Ozzy, the madman still in control. Ronnie James Dio performing CPR to save the day)
Ozzy Osborne, No More Tears (Proof that, even when out of control, a madman still rules the night)
Genesis, Duke & Invisible Touch (Genesis shines when trying not to play the hits)
Uriah Heep, Demons and Wizards (Ken Hensley showed us he had a vocal range equal to, or greater than, Freddie Mercury)
Peter Gabriel, Us (variety of musical influences from around the world, distilled into a prog rock package)
Mike Oldfield, Tubular Bells (one man show, only 19 years old. Helps Virgin establish itself. Lays down the tracks when the studio closes for the day. Like, wow, man!)
Pink Floyd, Wish You Were Here (Floyd's most soulful. These guys were playing for themselves, allowing us to listen in as they worked through the loss of a friend)
Led Zeppelin, Physical Graffiti (hard rock, prog rock, middle eastern influences, acoustic rock, ballads from a band so tight that they were to soon call it quits when their drummer dies)