What is the best Discworld stand alone book?

So my book bestie and I have a book club where we each read a designated book and then call and chat about it for a half hour each week. We normally break the books up into manageable chunks (ie: read Chapter 1- 12 for the first week, then 13- 24 for the next, etc etc). We alternate who selects the book and I wanted my next selection to be a Discworld book. I have read about 25 or so of the series including all the Death Series, Witches Series, and Tiffany Aching Series, and am currently reading the Watch Series with my son. So I was hoping to use one of the more stand-alone books since my friend hasn't read any Discworld books yet. Is there a specific one you would recommend and why?