Without in any way dismissing any previous suggestions otherwise, and despite what Terry Pratchett himself might say, I don't see a problem with simply reading them in the order they were written. After all, there are probably quite a few bazillion of us who read each one within days of its first release and have been quite satisfied with that order's experience.
Besides, the earliest ones are very short one-night-stands, so they're quickly filed on the bookshelves rather than in the TBR pile.
Cheers,
Marc