I've always remembered this as the best Discworld book, and so does nearly everybody else. And I really wanted to mark it down, both because it's always good to disagree with a consensus, and because early on in the book I got really quite irritated by the unsubtleness and heavy-handedness of Pratchett's satire against religion. And I got pretty irritated later on by the unquestioning attitude the book takes towards Pratchett's own views.
But... I can't. Those things may stop it from being my favourite Discworld book, may stop it (as it might otherwise have been) from being my favourite of all books, but they're nowhere near enough to stop it from being the best Discworld novel and an absolute masterpiece. The charm and brilliance of the book will override the thematic problems for almost all readers (indeed, for almost all readers they'll be enough to make it so people don't even notice the thematic problems, which is a testiment to Pratchett's talents, but also a little scary - he could be a really dangerous propagandist if he took up with the wrong ideology, I think...) - and even if they don't, it's just exquisitely composed. It's fun, funny, moving, thought-provoking, and infinitely quotable.
It isn't the best book I've ever read, but it's certainly up in the top tier.My fuller thoughts can as usual be found over here on my blog.