Hmmm I'll start.

I think I would go for The Colour of Magic and The Light Fantastic. Not sure if I would call them underrated, but I don't think they have the same following as the later books. Maybe it's deserved - as we know, they were written 'by a younger, stupider author' or something along those lines. The world-building only began there, PTerry was not quite yet in his full-on god-like-genius-with-language-stride (just let me have that one..!) and it's more of an easy read, ligher kind of fun. Not quite so touching, or deep, or touching on so many important themes through the perfect mix of humour, sadness and kindness. Which I do agree with.

However, I have read them both as soon as they came out in my country, I think the Czech translation for both. And I remember reading and laughing out loud so much I cried, soooo many times. So did my father, who also read them around the same time. And my sister. The laughing out loud part throughout the entire book was not there in the later books, at least not for me. Of course I laughed at parts, and felt things at other parts, sometimes hopefully even when The Author planned. But not throughout the entire books anymore. And for that, these two stand out for me.

Now come on, someone, tell me why I'm wrong so we can get a nice conversation going!