Guards! Guards! is often mentioned as a good place to start. Earlier books are still good, but more unpolished. The first two books suffer most from this imo, they don't give you a great sense of Pratchett's style and they're more direct parody than later ones.
Brandon himself recommends starting with The Truth, Going Postal, or Guards! Guards! (https://www.brandonsanderson.com/goodbye-sir-terry/) I actually started with the first one (The Colour of Magic.) I read it age nine or ten and I've been hooked ever since.
Brandon also wrote an article about Terry's work, you might already have read it but I'll post it just in case. It's a great article and I recommend it to everybody, whether they've read his books or not.
Terry Pratchett’s Discworld Might Be The Highest Form of Literature on the Planet | Tor.com