My list leans a little more toward the electronica/ambient/space than it does alternative, however these are what I feel are essential discs (many mentioned here already) --
Eno - Apollo* , On Land
Steve Roach - Dreamtime Return*, Magnificent Void
Tangerine Dream - Rubycon, Stratosfear
Michael Oldfield - Tubular Bells*
Constance Demby - Novus Magnificat
Budd/Eno - The Pearl*
Harold Budd - Lovely Thunder*
Jean-Michel Jarre - Oxygene, Zoolook*
Talking Heads - Fear of Music
Eno/Byrne - My Life in the Bush of Ghosts
Laurie Anderson - Mr. Heartbreak
Peter Gabriel - Passion *
John Serrie - And The Stars Go With You*
Aphex Twin - Ambient Works, Vol. 2*
Philip Glass - Koyaanisquatsi *
Global Communication 76:14
Michael Brook - Cobalt Blue*
Biosphere - Substrata
James Johnson/Robert Scott Thompson - Forgotten Places
Sylvian & Czukay - Plight & Premonition*
Patrick O'Hearn - Indigo, Between Two Worlds, Ancient Dreams*
Popol Vuh - Tantric Songs/Hosianna Mantra
Pixies - Dolittle
Radiohead - The Bends, Kid A, OK Computer, Hail to the Thief
Massive Attack - Mezzanine
Moby - Play
Orbital - Snivelization
* Particularly well-recorded
Of course, I'm sure I've left off some gems. But there are more than enough here to keep one's ears busy for quite a while.
Cheers,
CD
