I don't think that The Diskworld Books are anything remotely like a series because there are so many subseries - the Guards, the witches, Susan, Unseen University, etc etc which might mention each other or not, and are often different in style and tone.
If I had my time over I would probably want to start with Guards, Guards. Or perhaps Mort. Or maybe Going Postal.
I'm as sure as I can be that different books are different, and will appeal to different readers. If I was asked where to start I'd mention my favourites, but stress that they are so different and that I think different people will like different books.
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