Re: I'm touched by the weirdness of this request...
I'll second those who say skip the first two (very much lesser) works.
After those, they fall into different periods. The earlier ones (the Witches, and the Rincewind ones) are the most directly funny, tending to thoughtful slapstick. My own startingpoint was the witches. Later he gets more humanistic and a bit darker: the peak of that would be Night Watch. And he did go downhill towards the end as the alzheimers set in.
Note that there are *lot* of literary and cultural references: if you're not English, you may get less out of them (but don't let that put you off). And allusions: the Pork Futures Warehouse described the financial crisis before it happened, while "Interesting Times" (published 1994) could have been about "9/11" in almost the same sense as Arthur Miller's Crucible was about McCarthyism.