The greatest ever as far as I'm concerned. I've always been a Beatles and Stones guy, but Bowie's consistent genius is unmatched. I'm currently embarking on a vinyl marathon. Finished Space Oddity, now halfway through the masterpiece that is Hunky Dory. Anyone who's never heard him should get their down to the local record store and pick up his collection (it's worth actually buying the vinyl/Cds, as you get to enjoy the fantastic cover art as well.)
Here's my favourite David Bowie song:
Other favourites include Moonage Daydream, Memory of a Free Festival, All The Madmen, Station to Station, We are The Dead, and Time. His cover of See Emily Play is also genius.