Hands-down, our family's favorite writer is Neil Gaiman. He writes for both adults and kids. My favorites of his books for grown-ups are American Gods and Neverwhere. My favorites of his kids' books are The Graveyard Book and Coraline.
He's also written a bunch of comics, including the Sandman series and Marvel's 1602 (which is open-ended for a sequel--Neil, if you should come across this, hint, hint?).
Also, he collaborated with Terry Pratchett on a brilliant book called Good Omens. You won't be disappointed.