"Well, I'm more lopsided than a one-legged badger. I think I'll have to settle for hunting stupid mice. I shall just wander up to them and sit on them until they surrender."
In Into the Wild, after Graypaw tells Firepaw that Ravenpaw managed to impress Tigerclaw. His reaction:
Firepaw: "What on Earth did he do to do that? Fly?"
Sure it was an incredibly depressing scene, but Tigerclaw's reaction to Graystripe's kits was hilarious.
Firepaw calling Darkstripe "Dirtstripe" behind his back in Into The Wild.
The New Prophecy
Cinderpelt echoing Yellowfang. "Do you think I talk just to hear my own voice?"
The scene where Tawnypelt first meets Crowfeather.
Tawnypelt: "Is that a WindClan warrior? Undersized specimen, isn't he?"
The running gag about prophecy dreams coming from fresh-kill in Midnight.
Midnight on why she prefers talking to cats instead of foxes or rabbits: "Fox talk is all of kill. Rabbit have thistledown for brain."
A Tribe kit proudly tells the Clan cats that he's going to be a cave-guard, but he has to be a to-be first. Crowpaw asks what a "tooby" is, and after the kit explains, Brambleclaw points out, "He means an apprentice. Like you." Crowfpaw snarls irritably and the kit goes, "You're only a to-be? You're way old!"
Brambleclaw's Keet tendencies coming out after he gets named deputy.
"He partially hoped to find a ShadowClan patrol just so he could tell them about his new position...if a mouse had crossed his path at that moment, he might have stopped it to tell it that it was about to be killed and eaten by the new ThunderClan deputy."
Power Of Three
Most of what Jayfeather says in Power Of Three, but especially these:
Jayfeather: Oh great. Let's lump all the useless cats together and hope a tree falls on them.
Jayfeather: I'm a medicine cat. If you want sympathy, go to the nursery.
Pretty much anything Purdy says.
Sandstorm informing everyone that when she becomes an elder, she'll be the crankiest elder who ever lived.
Shimmerpelt: Are we supposed to eat an omen, or should we go catch some more fish?
At the beginning of Rise of Scourge, Tiny runs out into the forest. When he comes back and tells everyone, nobody believes him. He quickly starts spewing Blatant Lies.
Tiny: "It's true! It is! Plus, I, I built a house out of twigs and leaves, and I lived in it! And then I caught a fox and ate it!"
In Cats of the Clans, Rock mocks tribe naming conventions.
Rock: According to tradition, [kits] are named for the first thing their mother sees when they are born, although to my mind that would lead to a lot of kits being called Roof of Cave or Wall of Cave or Floor of Cave.
Tiny's imagine spot in The Rise of Scourge when he lies about defeating a dog. It involves him and the dog fighting in a dust ball and hilarious Art Shift.
When Sharpclaw goes to report to Leafstar after dealing with some quarreling Clanmates only to awkwardly turn his head away once he realizes she's nursing her kits.
In After The Flood, take a look at Sharpclaw's face when Cherrytail tells him to relax about her helping the Clan while still pregnant.
Billystorm trying to hint to Sol that he wants a private discussion with Leafstar:
Billystorm: Ahem, Sol, don't you have a patrol to go on?
Sol: Not that I know of. Why? Do you want to lead one?
The entirety of the Code of the Clans story "Hunting Fish!". Special mention goes to Dappletail and White-eye being punished by being forced to eat fish, and Pinestar's line, "We are ThunderClan. We. Don't. Eat. Fish."
The reactions Neo BloodClan has to the dogs in The Heart of a Warrior. Snapper and Pounce have huge bulging eyes, while Willie just gives a nonchalant "seen it all before" look.
Before that, Brambleclaw (seen as an apprentice here for some odd reason) bashes two cats' heads together with a trolling smirk on his face. Then, when the dogs join the fight, he cheers "Let's go, dogs!"
And the look on Pounce's face when Graystripe knocks him upside the head. The moment is sealed by the "bell ringing" sound effect accompanying it.
This exchange from Cloudstar's Journey:
Apprentice 1: "Way to go, Cloudstar!"
Apprentice 2: "Seriously? 'Way to go, Cloudstar'? He's our leader, don't you know?"
Graystripe scares two kittypets pestering him in The Lost Warrior by threatening to eat them and sharpen his claws on their bones. They yowl in fright and flee, making Graystripe think "That wasn't as much fun as I'd thought."
This is more of one caused by an editing error than anything In-Universe, but: in Bramblestar's Storm, during Graystripe's retirement speech, he mentions how he'll never forget his two kits... Stonefur and Mistystar. This troper had to put down the book and laugh for a good five minutes.
When the camp floods in Bramblestar's Storm, the apprentices' mentors briefly shelter with them. Dewpaw's reaction to this is: