Discworld

This one from Pyramids:

The fact is that camels are far more intelligent than dolphins. They are so much brighter that they soon realised that the most prudent thing any intelligent animal can do, if it would prefer its descendants not to spend a lot of time on a slab with electrodes clamped to their brains or sticking mines on the bottom of ships or being patronized rigid by zoologists, is to make bloody certain humans don't find out about it. So they long ago plumped for a lifestyle that, in return for a certain amount of porterage and being prodded with sticks, allowed them adequate food and grooming and the chance to spit in a human's eye and get away with it.

Always loved the idea that animals are just making idiots of us and using us for their own purposes.

I played the Duke in a Stage adaptation of Weird Sisters and the whole cast wrote quote for their character in the front of their script. Mine was
“The duke had a mind that ticked like a clock and, like a clock, it regularly went cuckoo.”

“Fear is a strange soil. It grows obedience like corn, which grow in straight lines to make weeding easier. But sometimes it grows the potatoes of defiance, which flourish underground.” ― Small Gods

A close second is also from the Small Gods; the very dark

“There are hardly any excesses of the most crazed psychopath that cannot easily be duplicated by a normal kindly family man who just comes in to work every day and has a job to do.”

No specific quote, just a thought that Vimes has several times...If you'll do something bad for a good reason, you're that much closer to doing something bad for a bad reason.

"People believe they want justice and wise government but, in fact, what they really want is an assurance that tomorrow will be very much like today."

Vetinari (Feet of Clay)

I don't see this one come up often when talking about Pratchett's work, but it's the one I see every day out there in my life!

Not that underrated, but don't see it around as much as it deserves too. From The Truth:

"There are, it has been said, two types of people in the world. There are those who, when presented with a glass that is exactly half full, say: this glass is half full. And then there are those who say: this glass is half empty. The world belongs, however, to those who can look at the glass and say: What's up with this glass? Excuse me? Excuse me? This is my glass? I don't think so. My glass was full! And it was a bigger glass! Who's been pinching my beer? And at the other end of the bar the world is full of the other type of person, who has a broken glass, or a glass that has been carelessly knocked over (usually by one of the people calling for a larger glass) or who had no glass at all, because he was at the back of the crowd and had failed to catch the barman's eye."

The one that made me burst out laughing even after decades of reading Pratchett was 'Minge drinking'

But my fave was the repeated nesting footnotes on Fingers Mazda. You can hear Pterry's cackling through the page.