I usually recommend that everyone start Discworld with The Color of Magic, so they can experience that progression as unfolds. The added bonus is that I get the vicarious thrill of watching those people go from "Yeah, it was a cute book" to "Oh my god, you were right, these books are Big Ideas in disguise and it is amazing."

Although, the other half of me just wants to send them directly to Night Watch to experience the genius first thing.