Yotsuba trips and falls down the hill, smashing her head on a sheep. Both Yanda and Jumbo laugh at her, but Yotsuba just does a grumpy victory sign to Yanda. And then she apologises to the sheep, it attacks her with its head, and then Yotsuba punches it in the face.
When Yotusba is going to punch a cow to prove to Yanda that she's not a coward.
And every other chapter with Yanda. Like when he bit her ice cream. Or when he prank called her. Basically, their first meeting introduces the funniest rivalry ever.
THIS POOP IS DELICIOUS!
The section in chapter 2 when Fuka gets stuck in the Koiwais' bathroom window.
And then made fun of by Asagi.
The whole series is pure Crowning Moment of Heartwarming with a Crowning Moment of Funny filling, but if I must pick one, it's the Finnish translation of the cicada scene in the first chapter. Yotsuba says "Olen kaskas" ("I'm a cicada"), which isn't funny yet. The thing is, "kas kas" is a common way to say "is that so?", which is exactly what Fuka says next, making this an epicly lame pun. I must have been laughing for half an hour straight. It's like Kiyohiko Azuma wrote the line expecting the eventual Finnish translation.
This Troper has to say, in his opinion "Yotsuba & the Stars" has some of the funniest stuff in the series. From her blinding herself with the "Yotsuba Beam!" and stumbling backwards into a trash can, blaming Yanda, to Koiwai asking Yotsuba if she knows any stars:
Koiwai: Are there any stars you know, Yotsuba?
Yotsuba: There is!
Koiwai: Oh, which one?
Yotsuba: Um... that bright, pretty one.
*Yotsuba points up at the sky*
Koiwai: Which star is it?
Pop quiz! What's something you go see, is large and round, and flies through the air?
When Yotsuba watches too many gangster movies and goes on a water pistol-toting rampage. From squirting down her own father in cold blood, to taking out one of Ena's teddy bears (while sparing Ena because she doesn't kill innocents), to the part where Asagi gets the drop on her and gives her a taste of her own medicine.
Sorry, but all of the above are subordinate to the absolute KING of all CMOF's: Jumbo going berserk when he finds out Miura is going to Hawaii.
Jumbo: And now our little princess will perform a hula dance!
Ena:<clap> <clap> <clap>
Miura: STOP THAT!
Chapter 67: Yotsuba finds an electric razor and immediately tests it out on Jumbo's beard. Jumbo is horrified.
Pretty much the entirety of Chapter 69 pt 2, but special mention goes to Miura's little attempt at wisdom on the first page:
Miura: Robot or not, all living things must die
Miura: Even though he's not a living thing...Since he's a robot...
Yanda's arrival in Chapter 41 and Yotsuba's reaction:
Yotsuba: A nasty guy arrived!
Jumbo: Ah true, a nasty guy arrived.
Koiwai: A nasty guy arrived.
Yanda: ...Give me a break.
A little later, they take a break and Jumbo gets hamburgers. In an adorable, and hilarious, moment, Yotsuba maliciously eats hers at Yanda, who only had cup ramen.
Yotsuba drops her newly-acquired teddy bear from a hot air balloon. Lesson about taking better care of one's possessions? Not... quite.
Torako:(stunned)IT LANDED ON ITS FEET!
"DAMN YOU, YANDA!"
NEVER COME BACK.
Yotsuba playing with the tape measure, more specifically when she asks a distracted Fuuka to hold it while she gets enough distance and sends the tape flying. Dad was not happy.
This page. Never be Yotsuba's (or Duralumin's) enemy.
This particular translation of a certain page (read the comments section).
When Yotsuba is shopping for ramen, she says, "With this much, I figure even if I ate it once a week, I could still have it every day." Another shopper has a hilarious look on her face as she tries to comprehend that sentence.
Volume 11 adds a bonus section to chapter 72, featuring Yotsuba, Dad, and Jumbo hiding when Yanda comes round. Badly. And then Yanda hides himself too.
There's also this:
Yotsuba: But Daddy ate so much that he turned into Full Belly Man.
Jumbo: What does he do when he's Full Belly Man?
Yotsuba: He doesn't move.
Chapter 76 is melancholy, as Yotsuba is lamenting over Juralumin no longer speaking. Fuuka decides she'll pretend to be Juralumin by proxy as a way to cheer her up. Yotsuba's "what the heck is your deal?" look is priceless.
Yotsuba and Ena pretend to be fancy ladies while camping:
Ena: Lady Yotsuba, dost thine eyes see what lies thither?
Yotsuba: Heavens, what could it be? My stars, it's a river! It looks ever so refreshing!
and then later...
Yotsuba: Is this where hillbillies live?
Miura: Heavens, that won't do at all. A lady of your stature saying such things.
Yotsuba: Goodness! You must forgive me.
Chapter 13 has Yotsuba stalking her prey, Fuuka's reaction to the frog, Koiwai's visitor, and the Reactive Continuous Scream from Miura, Ena and Yotsuba at the end.
In chapter 17:
Mr. Koiwai: Very cute! Just like the Little Match Girl. Yotsuba: Ahh, the one who sets houses on fire? Mr. Koiwai: No, not at all. Which one is that?