Although any individual book might only rate a 4, the whole discworld series is so consistantly funny and intelligent that it sets the standard in fantasy humor.
Pratchett satirizes every type of writing, from swords and sorcery, through detective novels(guards, guards) and shakespearian drama,(wyrd sisters) to coming-of-age and war stories(mort, monstrous regiment). He even takes a few pot-shots at science writing with the science of discworld.