Probably my favourite sub-series was the Witches and Nanny Ogg. As I caught up to the newer books I found myself enjoying them less because it felt like there was more and more philosophising. Probably what happened was that they got a bit more sophisticated and I was too immature to understand many of the themes and allusions.
Time to start working through them all then! I recently reread Colour of Magic and it was enjoyable but also clearly had far too much going on.
Either Pyramids or Moving Pictures was the only one that I really struggled to enjoy for some reason.