Howl hamming it up and covering the castle in green slime. "Despair! Anguish! Horror!"
Sophie's reaction. "If it wasn't for Calcifer, he could behave like a jellied eel all day for all I care."
Any of Howl and Sophie's bickering is funny, but there's also this, the mental picture is just hilarious:
Howl: If I were to court a girl who lived on an iceberg in the middle of an ocean, sooner or later - probably sooner — I'd look up to see you swooping overhead on a broomstick.
"I can sense Sophie's hand a mile off. And there are several miles of this suit." Later in the same scene (all of which is a Crowning Moment of Funny) "I am now," he announced, "going to get ready for the funeral. Please, both of you, refrain from doing anything whatsoever while I do. I can tell Sophie is in top form at the moment, and I want this room the usual size when I come back into it."
Sophie's family trying to clean the castle and complaining about its messiness when they come to visit, in the middle of Howl's battle with the Witch of the Waste.
Howl returning to the castle drunk, singing Calcifer's sauce-pan song. Later, after he's gone upstairs to bed, Sophie hears thumping, and him giggling about how the bed is "dodging" him.
Howl: "I assure you my friends, I am cone sold stober!"
Sophie's first experience with seven-league boots, where she marvels at traveling miles in a single step, but then can't stop.
There's Howl screaming down the stairway, running into everything while pulling at his hair, the green slime itself, and Sophie hauling Howl upstairs while he's covered in green goo... noticing only too late that his towel's slipped off, and he's completely nude.
Markl: He's calling up the spirits of darkness again! ...I saw him do this once before, when a girl dumped him.
Sophie: Well, now I have to mop again
From the same scene.
Howl: I give up. I see no point in living if I can't be beautiful.
Made even funnier in that it was Christian Bale saying that. Never knew, Bats, never knew...
The mention that Howl once reacted to getting dumped by calling up the spirits of darkness. like an wangsty teenage goth.
Sophie is asking Calcifer to let her use him for cooking, and the line that Calcifer says after she forces the pan on him:
Sophie: Let's get cooking.
Calcifer: I don't cook! I'm a scary powerful fire demon! RAWR!
Sophie: *shoves a frying pan on top of him, smushing him into the bottom of the fireplace*
Howl telling Sophie why the Witch of the Waste is after him. Sophie just makes a face that says "are you serious?"
In the ending, where Turniphead's curse is broken. It's the typical true-love's-kiss-breaks-the-spell, but...
Witch of the Waste: Looks like your true love is in love with someone else.
The scene when Sophie goes to see the king, and is trying to figure out what Howl disguised himself as. In the English version:
Sophie: *Sees a flying device with a girl shrieking and giggling with joy above* That could be him.
Just as Sophie and Howl escape Madam Sullivan.
Howl: Did you have to bring those two with you? (Sophie glances back and sees old!Witch of the Waste and Heen) Witch of the Waste: Nice doggy. (Heen wheezes) Sophie: I can't believe you work for Sullivan. I guess it's too late to toss them.
Sophie and the Witch of the Waste trying to climb up the stairs to Madam Suliman's castle. In other words, two elderly (one a teen trapped in an elderly woman's body, the other, a morbidly obese woman, who, until this point in the film, was the bane of the entire country) trying slowly and desperately to outrace each other to reach Madam Sullivan.
"You cold-hearted hag! Next time, I'll turn you senile too!"
The fact that while Sophie's going up the stairs, she's dragging a ridiculous-looking dog that she thinks is Howl in disguise. She spends most of the scene whispering to the dog, talking to it under the impression it's Howl. Even though it's difficult for her, she carries it up the stairs so she can keep him around, like he promised. Then, she finds out it's a real dog that works for Madame Sullivan.
"SHE LIKES MY SPARK!!!"
When Sophie introduces Turnip head to Markl.
Markl: Are you sure you're not a witch?
Sophie: I'm the worst kind of witch, the kind who CLEANS.