Good Omens, by Neil Gaiman and Terry Pratchett.
Its one of the few books that doesn't have the name "Adams" on the front cover that has made me laugh out loud. Gaiman's good on his own. Pratchett's good on his own. Together, they crafted a book greater than the sum of its authors.
Why'd I pick that one? I've had a loaner copy of it to give to random people for a few years now.