There's a lot of opinions on this. I found Small Gods to be a good jumping in place because it's a stand alone book and late enough that he had found the tone he wanted for the series. But a lot of other people recommend picking a subseries and starting with the first book there. The Vimes books are very popular so a lot of people recommend Guards, Guards as a starting point.
The reason a lot of people don't recommend publishing order is that the first two books are written in a very different style to the later ones. They're pretty straight parodies of heroic fantasy. But Pratchett becomes so much more later.
Witches aren’t bad to start it’s where I did. But I recommend ending with Tiffany Aching. The shepherds crown wasn’t intended to be the final book, he was writing until he died and would’ve kept going if he could’ve, but it is the perfect final book.
I’d say start with Rincewind, Witches, or Death. City Watch is good too but it’ll hit you hard with Industrial Revolution stuff and is very much the story of the world progressing as people try to deal with it.
You can do whatever I think, either read them by series (rincewind, witches, city guard, etc.) or by publishing order, starting with the colour of magic.