Greatest? I expect to change my mind six times between now and when I click "Post". But here are a couple of favorites from my pre-filk days:
SF: Ziggy Stardust -- David Bowie
Fantasy: Listen, Learn, Read On -- Deep Purple

(Close runners-up from T-Rex, Moody Blues, Uriah Heep, Queen....)

More practical, perhaps, are you familiar with this: http://www.textfiles.com/sf/sfmusic.txt