I definitely find myself inclined to the opinion that the last Discworld novel to be published was I Shall Wear Midnight. Which is not without some of the problems of the last three (there's a stiltedness to the dialogue sometimes that makes me struggle to read it, though it's nowhere near as bad there as later, and a certain amount of forecedness to the plotting, and it was the first one where I really felt that Pterry's illness was affecting his work), but is still generally pretty good.

Happily, my collection actually bears out this theory, as I only have ebooks of Snuff, Raising Steam and The Shepherd's Crown, but we have paper copies of all the rest.