I haven't read many of these, and I'm afraid I don't like them as much as I'm meant to -- shock, horror!
I first started reading them years and years ago, my best mate's brother recommended them. So I went out and bought the first two -- 'The Colour of Magic', and 'The Light Fantastic' -- sorry but I found them a little slow, and I wasn't very enthusiastic. That sort of put me off a little, although people compared him to Douglas Adams and raved about him.
Then I was bought 'The Last Continent' as a present. I really liked that and changed my mind. I also read his book 'Only You Can Save Mankind' which is very unusual.
Then my Mum and Dad produced the play 'Mort' at their amateur theatre and they raved about it too. I didn't think they would have liked it at all.
I have just finished 'Guards, Guards', which I did like but which took me forever to finish.
I do think he is clever in his analogies between the Discworld and our modern world, and I like the sense of humour, but the stories themselves take too long to develop. They aren't books that I must avidly read to discover the ending, just ones that I can pick up from time to time and have a laugh at.
I probably need to read more to make a decision on the best.