My success in getting people addicted to Discworld is directly related to whether I made the unfortunate oversight of giving them the first two books to start with.
Discworld is one of those series' that doesn't really have a set order. You can learn the basics from any book -- although I agree that everyone should eventually have read
all the books.
The important thing to start with is to not scare people off.
People expecting a book by "that humorous Fantasy writer" I recommended get the wrong expectations from the first books, and decide it isn't that interesting.