You have to start with The Colour of Magic for the discworld series. It's not really children's fiction but you get the energetic imagination of Pratchett on every page. Other than the Discworld series, I am bowled over by Truckers, Diggers and Wings. This trilogy is a parody of society seen through the eyes of Nomes, a hidden race of tiny people living in a department store. It is hilarious, and should be compulsory reading for EVERYONE!