I’ve grown up with Pratchett books and I’m still rereading them from time to time. I think my favorite would be either Mort or Small Gods. But there’s so many more worth reading. I particularly like the Witches arc… Wyrd Sisters, Lords and Ladies, Witches Abroad and so on.
Also, it’s not from the Discworld, but I think Nation is his best book overall. Most of the Discworld books are mainly poking fun at philosophy (in a very good way, don’t get me wrong) and say something really clever once in a while, but I found Nation to be a really profound look at how relationships, society, tradition and culture work. And that while still being a fun children / young adult novel. Really worth a read, even more so if you’re reading it with your kids.