Guys. Srsly. Srsly you guys.
We can't be friends anymore you guys.
Guys srsly.
(Actually, the problem is that with the series being as long and sprawling as it is, there are now, as per Sturgeon's Law, quite a few mediocre Discworld novels. "Mort," "Eric," honestly "The Colour of Magic" and "The Light Fantastic," "Going Postal" and "Making Money," "Carpe Jugulum.") That's just off the top of my head.
My personal favorite has pretty much always been "Reaper Man," though if you can overlook the fanboying of Captain Vimes and the need to have a solid grounding in the world to understand the references, "Thud" is really enjoyable. "Witches Abroad" was the first one I ever read, and it's still pretty high for me, though mostly because of Greebo turning into a human. Also I'll definitely second "Small Gods.")