I, too, love Good Omens, having read much of both Gaiman and Pratchett. While I wouldn't go as far as to say my favorite book, it does nonetheless have an awesome quality to it that makes me go back and read it every once in a while. Pratchett's Discworld series also has some great characters (Sweeper
) and storylines as well, though I think Gaiman added a lot to Good Omens that is missing from Pratchett's other works. (Don't hate me for saying it just 'cause we're on a Terry Pratchett thread... I can't even rightly place what it is that I think is missing.)