If they're willing to try a few books, I would just start them off at the Colour of Magic.
However, if they want to commit to only one book, I would suggest one of the stand-alones to them and if they enjoyed that, then point them to CoM.
It's been a while since I last read the books but Thief of Time has always stood out to me as one of my favourites. To me it's a great introduction to the deeply ludicrous scenarios that Pratchett could write. I would recommend that one as a first stand-alone.