The whole book. "Nobody remembers the singer. The song remains."
Lonard Da Quirm. One sentence.
I have no use for people who have learned the limits of the possible.
Rolling a seven on a six-sided die through very, very careful use of a sword, and thusly cheating Fate, is a Crowning Moment of Awesome that resulted in Cohen outdoing himself permanently. It was just that awesome.
Not only did they cheat Fate, using a million to one chance, when Luck tells them it was her because she is the million to one chance, they tell her to go screw herself, because she might be the million to one chance, but she's all the other chances too. Approaching, but not including zero, which is probably Fate. Just the fact that they call out the Lady, who usually plays the protagonist in the games the gods play and so is seen as pretty okay, who literally just saved their asses at dice, because even if she's usually playing the good side SHE'S STILL PLAYING DICE WITH THE LIVES OF MEN.
And Cohen's reasoning for calling her out? He's seen far too many good men take a Million-to-One Chance... and fail. The Last Hero gives the woman who screwed over all the others the finger.
The Silver Horde's final scene. After they save the world from themselves, the Valkyries come to take them away to the heaven they've always expected... but they don't want to go, so they steal the Valkyries' winged horses and head off into the universe. After all, they don't think they're dead, and when have they cared what anyone else thinks?
And a side note, there's five of them, but they take all seven horses, I presume the other two are for Teach and Vincent.
A small one for Vena, too, because she successfully impersonates a valkyrie.
A guide to the Discworld novels that was published for a time with paperback editions of the novels says it best: "In one of the world's most dangerous professions (barbarian warlord), he has lived to be a very old man. Think about it!"
When the crew of Kite first saw one of the Elephants with THEIR OWN EYES & Discrise (while on Moon). Seeing is believing, huh?
Rincewind's suggestion for the badges in The Last Hero: Morituri Nolumus Mori.
Vetinari: And would you care to give us a colloquial translation, Mr Rincewind? Rincewind: Er...er...roughly speaking, it means, "We who are about to die don't want to", sir.
Rincewind managed to flummox Vetinari with his (perfectly valid, for him) reasoning for why he'd volunteered against his will.
A short one: Mazda (the Discworld Prometheus), freed from his chains, works out the circumstances:
He was aware that he was holding a very sharp sword. And he could hear, with the rising sun, the beat of an eagle's wings. He was going to enjoy this.
The whole idea and pun behind “returning fire to the gods”.
At the end of the book, the gods order Leonard da Quirm to paint the ceiling of an enormous temple in Ankh-Morpork with suitably epic art, within ten years. Those present remark that this is a very short amount of time for such a task. But three weeks later, he has finished. He finishes a ten year task in a week. And what did he paint? He painted the Discworld itself, as seen from directly above, from the moon.
Not only that, but he did all the painting without any scaffolding. By building a miniature helicopter and an airbrush.
When it had been his dream for multiple books to make a flying engine.
Carrot taking a stand against the entire Silver Horde, armed with nothing but his non-magical sword... ...and the Horde pausing, remembering that on Discworld, thanks to the Laws of Narrative, one brave, determined man can take on a group of dangerous warriors, and win.
The second picture of the bard—fey and wild, playing on a skull harp and crying.
Paul Kidby gets a meta-one for the art, especially how he faithfully represents all the wonders of the Edge of the Discworld from space as mentioned in The Colour of Magic, including the Krullian Circumfence, the Rimbow and the Rimfall.
Rincewind managing to pull a lever while going at full speed down the edge of the world.
During the meeting of Ankh-Morpork's leaders, the Dean of Unseen University protests against Leonard's invention, stating that they have no assurances other than his word that it will work, citing Bloody Stupid Johnson as an example of the unreliability of inventors;
"Show me one thing he can do that anyone couldn't do, if they had the time."
"I have never considered myself a genius," said Leonard, looking down bashfully and doodling on the paper in front of him.
"Well, if I was a genius, I think I'd know it-" the Dean began, and stopped.
Absent-mindedly while barely paying attention to what he was doing, Leonard had drawn a perfect circle.