Sad to say, I've read my way through the Pratchett Discworld oeuvre since stumbling on The Colour of Magic and The Light Fantastic a long long time ago. IMO the ones that feature Sam Vimes' vain attempts to keep order in Ankh Morpork are the best (not so keen on the witchy ones). So any of Guards! Guards!, Men at Arms, Feet of Clay, Jingo, The Fifth Elephant, Night Watch or Thud!. And since I'm in confessional mode here, I'll add that I reread Feet of Clay as soon as I'd finished it. PS I've seen Pratchett described, by a broadsheet reviewer no less, as 'silliness with A-levels'. Though that's probably not such huge praise anymore.