My favourite Discworld novel is tough to pick, but I'd have to go with Night Watch. The story and the characters are so engaging, and Pratchett's trademark satire is on full display with some added depth to it. It's a wonderful mix of fun and thought-provoking commentary.
The depth of the world building in Discworld is incredible; every time I reread a book, I spot something new! The attention to detail and the intricate connections between different story arcs are masterful. Pratchett left a huge fantasy world legacy, which I think will continue to captivate readers for a long time.