More or less any moment involving the Literature Girl.
Episode Five's internal-Big "NO!" moment, when poor Hidenori realises she's standing right behind him again. The screaming is perfect.
Episode Seven is one long Humiliation Conga for the poor girl. In order of events: (1) the wind blows away the manuscript of her novel, (2) when she starts chasing after it she trips and falls and bruise her hand, (3) she struggles to open an box of bandaids for her bruise before finally giving up, (4) when trying to drink a bottle of water to regain her composure she suddenly starts choking on it, (5) she suffers a choking fit and ends up blowing a large amount of snot out of her nose, (6) she ends up tripping and crashing into Hidenori, and finally (7) she starts bawling her eyes out after the string of humiliation. The entire time, Hidenori can barely contain his laughter. When he tries to reassure her with encouraging words, he ends up accidentally headbutting her and knocking her out.
Episode Ten's hilarious Chase Scene involving the two of them. And the reason she wanted to chase him down so badly? To yell at Hidenori that the guy she was with when he saw her wasn't her boyfriend.
In "High School Boy and Drop Kicks", Hidenori meets a girl who dropkicks him because she thought he was her friend Kiyohiko. When he actually comes by, Hidenori tells him to take action and take payback for all of the times she's kicked him, and then accidentally throws them both into the river. Kiyohiko's response to any of this? He cries and tells his friend that no one has ever said that before, and instantly befriends Hidenori. The girl is left wondering if this entire scenario was "Boy Meets Boy".
At the end of the summer, the girl turns out to have fallen in love with Hidenori, and tries to confess to him. Before she can, he mentions something very surprising to her— after visiting his grandmother, he found out that he and the girl are actually cousins. Cue Freak Out.
The last main skit of the anime, "High School Boys and...", while being nothing short of a Crowning Moment of Heartwarming for tying up most loose ends after the main crew graduate, ends with Tadakuni waking up after seeing Nago become the "Convex Mirror Girl" again, revealing that the entire segment was a dream.
The Once an Episode credits gag for the anime, where a picture of Yoshitake holding a tie increases in number starting from episode 2 (and the numbers somehow correspond to the number of the episode). There are episodes that particularly stand out:
The ninth episode features a parody of Andy Warhol's art pieces, having nine Yoshitakes multicolored and positioned.
The show goes on to make a 10 by 10 square of multicolored Yoshitakes for the tenth one.
On the eleventh, they only take one Yoshitake, but they blow it up to fit half of the screen.
On the twelfth and final episode, they instead position a number of default colored Yoshitake pictures to make the word "END".
Who could ever forget this one where Hidenori and Yoshitake dressed Tadakuni with his sister's clothes in 'Highschool Boys and Skirts' when she went to the room and saw the ordeal. Cue Freak out from the boys.
In "High School Boys and Mochi Soup", where the three protagonists all happen to be choking on mochi despite the content warnings.
Cue Tadakuni getting punched in the stomach by his sister.
Their wacky dubbing at "High School Boys and Dubbing"