From the ones already mentioned:
Neil Gaiman
Douglas Adams (except for book 5 of the Hitchhiker series)
Kurt Vonnegut, jr.
Stephen King (even though he's admitted in interviews that he makes up all of his stories as he goes along)
Ursula K. LeGuin
J. K. Rowling (forgiving the Disney Ending)
And for novelists who haven't been mentioned yet (which is a crime), look up:
E. L. Konigsburg, who can use subtlety like no other author (I challenge anyone here to read Father's Arcane Daughter and claim that it's not a masterpiece)
Michael Chabon. The film adaptation of Wonder Boys is a woefully under-appreciated little gem, but his real masterwork is The Amazing Adventures of Kavalier & Clay (which will have a special significance to fans of a magazine called "The Escapist").
Peter S. Beagle, best known for The Last Unicorn but I like his other stuff even better. I of coure have to mention him because he's a friend of my father's, which makes it a little awkward recommending his work, but he's still a master.
Which reminds me, I feel obligated to also recommend Into the Light, by Alex Hancock, because... well... the author is my dad ^_^; (in all seriousness, it's actually pretty good)
Writers in other mediums (the OP said playwrights and stuff were welcome too):
William Shakespeare (obvious choice, but still)
Joss Whedon (ATTENTION: if you think that all Joss Whedon can write is one-liners, I have an assignment for you: Watch episode 16 of season 5 of Buffy the Vampire Slayer (the episode entitled "The Body"). You don't have to have seen any other episodes of the show, just rent the DVD and watch that one episode from start to finish. That's an order.)
Steven Moffat, and YEAH!!! [http://www.guardian.co.uk/media/2008/may/20/bbc.television2] And double yeah [http://www.comingsoon.net/news/movienews.php?id=37909]!
Jeff Smith. I think the Bone comics may be the greatest comic book series by a single author ever. Both the story and art are phenomenal, and he kept the series going singlehandedly for ten years. Ten. Years.
Yahtzee Croshaw (sorry)