The consensus from this thread
seems to be that the books can be read in pretty much any order. I can't comment on that as I have read all the books in order and can't imagine going through the series in a random order.
For stand alone books I would have to say Pyramids, Small Gods and Monstrous Regiment stand out in my mind, with Amazing Maurice being included as well.
I think though, it's safe to say that, once you have worked out how things work on the Discworld, you shouldn't have too much of a problem.
"Goodness is about what you do. Not what you pray to."