[*Maybe because I have just reread it.... Witches Abroad.
It suits the money making formula alot of "kids movies" have these days of entertaining the kidlets immensely with the fairytale references they would pick up on, and it would amuse the adults with all the jokes that would go completely over the kids heads.
Well, they'd have to leave out the killing of the suffering wolf that was too human...
Though not a good introducer to people who don't know the discworld characters, I'd love to see "Lords and Ladies" on big screen. I adore the whole story from page one to the very end.
So I'd go for "Guards! Guards!", because of the dragons, and the good introduction of the watch series. Jeremy Irons as Vetinari is perfect, me thinks. But who indeed would play Vimes?