I liked the ones about the 3 witches "Equal Rites" being my favorite. I also have read "The Wee Free Men" and enjoyed it very much, there's a sequel now apparently too I must get it as I liked the new characters.
My favourite is probably Guards! Guards!, although I have enjoyed all of the Discworld books.
I liked the book he did with Neil Gaiman, Good Omens, as well, although not as much as the Discworld books.