My favourite are Jingo, Hogfather, Pyramids, Thief of time (half - the bits with Lu-Tze and Susan), Going Postal, The Truth, Interesting Times, Last Continent, Guards Guards, Nation and Good Omens.
My least favourites are Monstrous Regiment (too depressing), Making money (silly ending), Wintersmith (Tiffany was stupid) and Maurice (not quite sure why I'm not too keen on it).
(Ooh! A touch controversial
The ones not mentioned are merely excellent!