Hmmm. Until I read Pyramids two months ago, this one would have been a no-brainer. Then I met You Bastard ...
OK. This may be slightly unfair, since Granny simply gets more page time than You Bastard, but:
1. Granny Weatherwax
2. Runners-up, on an equal footing:
* You Bastard
* The Librarian
Total badasses and hilariously funny, the lot of them -- and all of them can expose humanity's (and society's) utter inanity more effectively with a single word, facial expression or gesture than most other characters even by Pratchett himself.
ETA: AND the Librarian of course, for all of the above reasons. How could I possibly forget the awesome Librarian of Unseen University?!
(Task: Who is your favorite Discworld character and why?)