Arthur C. Clarke, 1917-2008. He's gone to that great black monolith in the sky...
It's difficult for me to say which one of Clarke's works is my favourite. Everybody knows 2001, but I maintain that owes at least as much to Kubrick as it does to Clarke. Childhood's End is a masterpiece -- probably the best writing that Clarke ever did, as writing goes -- and blew my frickin' mind when I was 14. Earthlight is a weird and fun little mystery that also happens to have the best final 50 pages of any golden age SF book that isn't Childhood's End or Canticle for Leibowitz (not Clarke, but still.) But I'm also a huge fan of Clarke's shorter works. The Sentinel which became 2001 actually managed to be more mysterious than the movie, I think, and then there's The Star, in which a Jesuit astrophysicist is part of a team of interstellar archaeologists who discover an ancient archive from a civilization that died when their sun went nova. The team spends weeks poring over the archive, learning the alien's culture and accomplishments, and it's only at the end that Clarke has his Jesuit reveal what his measurements have proved: this star went nova, destroying a beautiful culture, so that the heavens could announce the birth of a baby boy in Bethlehem, c. 1 A.D.
My teenage mind: ka-BOOM.
There are precious few SF writers who've had the kind of impact that Clarke had, and it's difficult to feel like a 90 year-old man got taken from us too soon. Still, somehow it feels like a slightly smaller universe with him gone.