If you only had to read one subject then I would choose the City Watch novels. These tell you much about Ankh Morpork but little about Discworld as a whole If you want to know about that, then I would suggest the Rincewind stories. Death and Witches are nice diversions, and then there are the novels that could easily stand alone.
Though in all honesty (and as I mentioned previously) , I really see no reason not to read them in publication order.
The only thing I would say is that there are certain stories which can be omitted, which (to me at least) aren't of the highest standard, and whose omission will not detract from your enjoyment or understanding of the Discworld series. Personally I'm not a fan of of some later stories such as Monstrous Regiment, The Thief of Time and Unseen Academicals. And I'm not all that keen on Going Postal or Making Money. I think I preferred Terry Pratchett when he stuck closer to fantasy themes than the real world. From a personal point of view, I think that he peaked with Night Watch.