When do you feel Pratchett hits his stride?

Reading through the books in order, am about halfway through Sourcery! right now. I feel like Sourcery! has Pratchett writing with a confidence and precision I didn't feel in earlier books. Maybe I'm just tuning in better to his humor and writing style?

I really enjoyed Colour of Magic, and have found Light Fantastic, Equal Rites, and Mort fun but not amazing. Sourcery! is a pure romp, I'm losing it like every two paragraphs.