What's your favourite Discworld quote?

My top two that I can think of

“And sin, young man, is when you treat people as things. Including yourself. That’s what sin is.”
"It's a lot more complicated than that--"
"No. It ain't. When people say things are a lot more complicated than that, they means they're getting worried that they won't like the truth. People as things, that's where it starts."
"Oh, I'm sure there are worse crimes--"
"But they starts with thinking about people as things..."

-Granny Weatherwax, Carpe Jugulum

It all starts with thinking about people as things

All right," said Susan. "I'm not stupid. You're saying humans need... fantasies to make life bearable."

REALLY? AS IF IT WAS SOME KIND OF PINK PILL? NO. HUMANS NEED FANTASY TO BE HUMAN. TO BE THE PLACE WHERE THE FALLING ANGEL MEETS THE RISING APE.

"Tooth fairies? Hogfathers? Little—"

YES. AS PRACTICE. YOU HAVE TO START OUT LEARNING TO BELIEVE THE LITTLE LIES.

"So we can believe the big ones?"

YES. JUSTICE. MERCY. DUTY. THAT SORT OF THING.

"They're not the same at all!"

YOU THINK SO? THEN TAKE THE UNIVERSE AND GRIND IT DOWN TO THE FINEST POWDER AND SIEVE IT THROUGH THE FINEST SIEVE AND THEN SHOW ME ONE ATOM OF JUSTICE, ONE MOLECULE OF MERCY. AND YET—Death waved a hand. AND YET YOU ACT AS IF THERE IS SOME IDEAL ORDER IN THE WORLD, AS IF THERE IS SOME...SOME RIGHTNESS IN THE UNIVERSE BY WHICH IT MAY BE JUDGED.

"Yes, but people have got to believe that, or what's the point—"

MY POINT EXACTLY.

-Hogfather

I have to add this one:

A man who could afford fifty dollars had a pair of boots that'd still be keeping his feet dry in ten years' time, while a poor man who could only afford cheap boots would have spent a hundred dollars on boots in the same time and would still have wet feet.

And this: https://en.wikipedia.org/wiki/Boots_theory

It is always useful to have an enemy who is prepared to die for his country. This means that both you and he have exactly the same aim in mind.

(From Jingo)

Also, the obligatory link to the Annotated Pratchett File which contains annotations for many of the obscure references and jokes throughout the series (and sometimes even additional commentary from online posts made by Pratchett himself).

I have a whole list of them in my quotation database. Many have already been mentioned here. One notable one is not from a book, but the man himself:

J.R.R. Tolkien has become a sort of mountain, appearing in all subsequent fantasy in the way that Mt. Fuji appears so often in Japanese prints. Sometimes it’s big and up close. Sometimes it’s a shape on the horizon. Sometimes it’s not there at all, which means that the artist either has made a deliberate decision against the mountain, which is interesting in itself, or is in fact standing on Mt. Fuji.

~ Sir Terry Pratchett

‘That’s a harp he’s playing, Nobby,’ said one of them, after watching Imp for a while.
'Lyre.’
'No, it’s the honest truth, I’m-’ The fat guard frowned and looked down.
'You’ve just been waiting all your life to say that, ain’t you, Nobby,’ he said. 'I bet you was born hoping that one day someone’d say “That’s a harp” so you could say “lyre”, on account of it being a pun or play on words. Well, har har.’

I have too many favorites, so these are more ones I remember offhand than favorites because they're all my favorite.

¹ This is very similar to the suggestion put forward by the Quirmian philosopher Ventre, who said, 'Possibly the gods exist, and possibly they do not. So why not believe in them in any case? If it's all true you'll go to a lovely place when you die, and if it isn't then you've lost nothing, right?' When he died he woke up in a circle of gods holding nasty-looking sticks and one of them said, 'We're going to show you what we think of Mr Clever Dick in these parts . . .'

“Some humans would do anything to see if it was possible to do it. If you put a large switch in some cave somewhere, with a sign on it saying 'End-of-the-World Switch. PLEASE DO NOT TOUCH', the paint wouldn't even have time to dry.”

“That is because you don't yet know how to deal with time," said Wen. "But I will teach you to deal with time as you would deal with a coat, to be worn when necessary and discarded when not."
"Will I have to wash it?" said Clodpool.
Wen gave him a long, slow look.
"That was either a very complex piece of thinking on your part, Clodpool, or you were just trying to overextend a metaphor in a rather stupid way. Which, do you think, it was?"
Clodpool looked at his feet. Then he looked at the sky. Then he looked at Wen.
"I think I am stupid, master."
"Good," said Wen. "It is fortuitous that you are my apprentice at this time, because if I can teach you, Clodpool, I can teach anyone.”

LORD, WHAT CAN THE HARVEST HOPE FOR, IF NOT FOR THE CARE OF THE REAPER MAN?
(from Reaper Man)

Oh fun. I re-read Going Postal last fall, and now I'm going through the Tiffany Aching books as a read-aloud with one of my daughters. They're new to both of us, and we're in the middle of Wintersmith right now; she's about the same age as Tiffany in this book, so that's a ton of fun. (At some point, I really need to skim back through and add more Tiffany Aching stuff to my quotations file.)

I can't choose just one, so here are a few favorites from my file:

Since I work in tech, love math and physics, and currently help design hardware...

What was magic, after all, but something that happened at the snap of a finger? Where was the magic in that? It was mumbled words and weird drawings in old books, and in the wrong hands it was as dangerous as hell, but not one half as dangerous as it could be in the right hands. The universe was full of the stuff; it made the stars stay up and the feet stay down.

But what was happening now... this was magical. Ordinary men had dreamed it up and put it together, building towers on rafts in swamps and across the frozen spines of mountains. They'd cursed and, worse, used logarithms. They'd waded through rivers and dabbled in trigonometry. They hadn't dreamed, in the way people usually used the word, but they'd imagined a different world, and bent metal around it. And out of all the sweat and swearing and mathematics had come this...thing, dropping words across the world as softly as starlight.

(Going Postal)

...and previously worked on the technical side of movie VFX.

That was how it worked. No magic at all. But that time it had been magic. And it didn't stop being magic just because you found out how it was done. . . .

(The Wee Free Men)

Try new things...

"Look," said Moist, "I don't know what's happening here, but I don't know anything about delivering post!"

"Mr. Lipwig, this morning you had no experience at all of being dead, and yet but for my intervention you would nevertheless have turned out to be extremely good at it," said Lord Ventinari sharply. "It just goes to show: you never know until you try."

(Going Postal)

...and aim high.

He always raised the stakes, automatically. Never promise to do the possible. Anyone could do the possible. You should promise to do the impossible, because sometimes the impossible was possible, if you could find the right way, and at least you could often extend the limits of the possible. And if you failed, well, it had been impossible.

(Going Postal)

"All the shops have been smashed open. There was a whole bunch of people across the street helping themselves to musical instruments, can you believe that?"

"Yeah," said Rincewind. "...Luters, I expect."

Recently read The Light Fantastic, and this killed me. So many potential choices for the best though.