least favourite?
Carpe Jugulum which I read once, and haven't been back to. I didn't spot the piece about it being a repeat of Lords and Ladies, but I can see the point.
Thief of Time, which I really couldn't get into. Another once-only book. The History Monks really aren't a good enough joke for a whole book
Lost Continent, another once-only which seemed particularly disjointed.
I Shall Wear Midnight, which appears to have a dose of Niven-itis ( trying to hard to join up things which were originally quite unconnected ) and the Feegles/ Wee Mad Arthur joke is overdone.
Another vote for Reaper Man, where the Windle Poons/ shopping trolley subplot seemed to have no connection with the much better "Death and Miss Flitworth" plot. Death, Miss Flitworth and the Crowned Death is really a good enough plot to stand alone.
I'm not a great fan of the Tiffany Aching books, although the rescue from fairyland is good - the elves are always worth an appearance for me.
I read the "nomes" books when I was younger and liked them at the time, so enough said. Nation doesn't work at all for me. I've never read Dark Side of the Sun etc and I doubt they would work for me at this stage.
Personally, I like Pyramids, it's several good jokes rolled into one, especially the sub-plot about the innumerable contradictory gods, and the quite ingenious time-paradox sub-plots involving Dios, the building contractor and the indefinitely-undying kings in their pyramids.
Maskerade is another favourite, again not really a Discworld book in some ways but very funny.