Almost impossible to choose! Love so many of them... I'd probably have to say (and trying to be sort-of equal with distributing my favourites between the different series)...
- Night Watch (and, to be honest, EVERY Vimes-centred book)
- Fifth Elephant (gets a mention for the sake of Carrot and Angua, who barely featured in NW)
- Reaper Man (Death, Reg Shoe... everyone. Touching, hilarious, amazing)
- Thief of Time (Lu Tze. Just... Lu Tze)
- Lords and Ladies (Magrat with an axe, Granny vs. the Queen... hello? Obviously going to be amazing)
- Small Gods (theology, it's fabulous :D Some bits were scary, though O_o And sad for their basis in fact. Other parts, so very funny...)
OK, I'll stop myself there. But there are quite a few others. *sigh*