Zatch/Gash and Kiyomaro are chasing after a nun's partner a rabbit/monkey looking Mamodo boy, Momon by name. Kiyomaro encounters him by the river bed where he pants him. When the Zatch/Gash gather they go looking for him, or rather he finds them and lift Tio's skirt. Not only does he get to look at her undies once, but twice. When they capture him she breathes a sigh of relief that Kiyomaro and Zatch (who the former she has a crush on and latter may or may not) did not see them.
From the dub: Kiyo sees that the river is frozen in the episode where the Ice mamodo Reycom appears. Zatch remarks that the river is filled with 'fish popsicles'. The look on Kiyo's face when he screams "FISH POPSICLES???!!!" is priceless. In fact, a lot of facial expressions from both the anime and the manga are priceless.
Konjiki No Gash Bell: Take your pick. Folgore's "Chichi wo Moge"/"Hey Hey Let's Dance All Day" moments. Just about anything Victoream says ("Very melon!"). Actually, Folgore and Victoream together usually result in insane moments of hilarity, and it happens twice on the show, the latter being an entire episode where Folgore talks about a movie he is planning to make about Victoream.
The chapter/episode where Kiyomaro goes camping. Just... the whole thing...
The simple fact that Papipurio made it to the top 10. The rest of the top 10 consists of the main villain+ henchmen, the protagonists and their worf friend, and then...Papi.
Bari's first encounter with Zatch. Specially because he thought that the weeping child who was getting pursued by a little girl couldn't be the one he heard rumors about so he assumes Naomi has to be a mamodo.
Danny's first encounter with Zatch as well.
Somewhere between this trope and CMOA is the greatest demon head of all time, which Kiyo takes to astonishing levels when he returns from the dead and curbstomps Rodeux with little effort.
During the first filler arc episode where Shion Hibiki, a new transfer student is introduced, her cat which she brought to school pops out of her bag, the teacher tells her off for this and says she can't bring a cat to school. Her classmates are quick to jump to her defense, and one of them points out (pointing directly at Kiyo) that "there are even those who bring children to class." One then mentions the teacher often brings his wife over to school. Cue said wife peering in from the door entrance.
The teacher allows her to bring the cat to school with that. Next day, everyone who got a crush on the new student tries to one-up Kiyo and brings to school either an animal (including a horse and stealing Ponygon from Kiyo's house) or a young child (including Naomi) predictably, the whole classroom turns into a mess.
In the following episode there's all of Shion's laughably inept attempts to take Zatch's MacGuffin, which culminates in a scene in the caffeteria that can only be described with "Everyone, protect your Yellowtail!"
Dr. Riddles bringing Big Boing along to an exposition drop for the sole purpose of trolling Kiyo by having her show him her new dance. Watching Riddles crack up is hilarious enough even before Kiyo's (predictable) response:
At one point in the Manga, Ruupa and Papipurio come across Ted and Jido inside of Faudo. They try to play dumb when asked what are they doing there. Ruupa pretends she's an alien by doing a bizarre, but hilarious impersonation of ET.
Kiyo's reaction when he walks in on the man in the fish costume.
"Just so you know, I don't usually do this kind of thing."
Tia learning a new spell out of anger at a mamodo that kept touching her panties.