This is probably my favorite book. That's a difficult decision for me to make - I'm plagued by commitment issues. But "Good Omens" proves itself, time and time again, to be excessively fantastic.
I first read this book in junior high school, and it was my first exposure to either author. I immediately went around trying to convince everyone I knew that they had to read it, too. The dry humor sang to me. The way the different plot lines wove together delighted me. The footnotes - o! the footnotes! - titillated me.
I've since read enough of both Gaiman and Pratchett to recognize their individual strengths in "Good Omens". They do complement each other. Gaiman's myseterious otherworldliness, Pratchett's sharp goofiness - it's like wandering in an eerie spectral fog and happening upon a purple wallaby taking a bath in a flowered teapot.*
This book will make you reconsider everything you've ever thought about the end of the world.
*The wallaby's name is Bruce, by the way, and he does wish people would knock before barging in.