Like many, the Guards and UU books were what got me hooked, but lately I've been leaning towards the Lancre stuff. However, Jingo does have one of my favorite quotes:
“It was so much easier to blame it on Them. It was bleakly depressing to think that They were Us. If it was Them, then nothing was anyone's fault. If it was us, what did that make Me? After all, I'm one of Us. I must be. I've certainly never thought of myself as one of Them. No one ever thinks of themselves as one of Them. We're always one of Us. It's Them that do the bad things.”