Always glad to turn people on to great literature. After the Crowley site you gave me the other day, I was getting ready to give Good Omens yet another read, but I wasn't aware that he'd just come out with another book, so it looks like I've got some reading to do.
While I'm inclined to jump to saying that Good Omens was my favourite (Terry Pratchett is also a great author, with his Discworld series now running somewhere in the region of 30 books), they're all good, and Neverwhere and American Gods are also favourites of mine. I also picked up a book of short stories by Neil Gaiman called Smoke and Mirrors (either when I was down in Michigan, or in NY) which I also wouldn't hesitate to recommend as a great read.
Cheers,
AJ