>>7
Mort, the 4th diskworld book, is where the series starts to get really good.
The best diskworld books are Night Watch, Going Postal, Small Gods and Reaperman. to get the most out of Night Watch and ReaperMan it is worth reading them in the order they come in the Watch series and death series but Small Gods is a stand alone novel and Going Postal is the first of the Moist series. If you're looking for a good stand alone Prachett novel Pyramids and The Truth.
He rarely has a novel that is on the flat side, the only ones that spring to mind are Monstrous Regiment (which was good up until the point where it seems he had to rush the book to get it finished by the publishers deadline) and The Light Fantastic (again a good enough book and a necessary one to conclude the events of COM but not as slick as COM was). That said, even those two novels are light years ahead of his competition and in any case only Neil Gaiman and Robert Rankin even come close.