Discworld

Basically, while all of the novels take place in the discworld, different books in the series focus on different groups of characters, making it so that there are different subseries within the series. A lot of people choose to read the subseries in order instead of going through the books in publication order. This is also because the first two books, while not bad, have a very different tone from the later books. They're kind of straight parodies of heroic fantasy rather than being more focused on the unique setting and characters within the discworld.

So most people would recommend starting with one of the major subseries. The city watch books start with Guards Guards (this is probably the most popular subseries). The witches start with Equal Rites. Death starts with Mort. And Rincewind starts with The Color of Magic. Or you could pick one of the stand alone books to get a taste of Pratchett's style without continuity. Small Gods is probably the best choice for this IMO.