Though the Last Continents jokes were somewhat predictable, it was still an enjoyable book. Really, the only discworld book I consider notably weak is Equal Rites, but he was still getting into his stride then: trying to find the transition between what could easily have been a one-off piss-take of Fritz Leiber, Robert E. Howard and HP Lovecraft (The Colour of Magic/The Light Fantastic, books which almost fit in better next to his comic sci-fi works like Dark Side of the Sun and Strata than they do the rest of the Discworld books) and the post-modern satire slash social commentary of the rest of the series. Mort is when the Discworld really comes into being, for me. Wyrd Sisters makes a much better introduction to the Witches anyway. There are variations in quality throughout the rest of course. I'd say my top five were probably Wyrd Sisters, Hogfather, Guards! Guards!, Mort and Jingo!, for a variety of reasons. The best thing about the Discworld books overall, though, was reading everything up till Carpe Jugulum when I was 11 or so, then re-reading them all again recently, now much more widely knowledgeable and better read. Reading the Discworld books at different comic levels is like reading two whole different series. The first time round I didn't get jokes like, for example 'Llamedos', or any of the Priscilla, Queen of the Desert jokes in The Last Continent.
I love Rincewind, actually. One reason to look forward to Unseen Academicals, for me, is his return. (For those not following the entire Discworld continuity, Rincewinds story has been continuing in the Science of Discworld books, and he is now permanently tenured at Unseen University as the Egregious Proffessor of Cruel and Unusual Geography, Assistant Librarian and Health and Safety Officer, among other things).