I reckon Guards! Guards! would be the best since it not only introduces the Watch but would also be one of the easiest for those not acquainted with Discworld to understand. To paraphrase (or misquote
) Sir Terry, "Its a story about cops. Just because I included a dragon, somebody decided that made it fantasy instead".
I took a friend, who had never even heard of Sir Terry (let alone the books), to the stage production starring Paul Darrow and she managed to enjoy it; despite normally having difficulties following the plot of a Star Trek episode.