Surprisingly, the flashback of how Sumika and Nanami met each other.
The chapter where Manabu recalls a memory of him witnessing his grandpa help out a woman in an abusive relationship after she comes to their house for help. The woman didn't even really know him and only came to him for help because she knew of him from other people, yet he still helps her out without a word of protest. It really makes you understand why Manabu has such an enormous amount of respect for his grandpa.
The lengths that Sumika goes to snap Nanami out of her Heroic B.S.O.D. due to being rejected by Manabu. She's willing to go to Reina, who she's not fond of, tell her about what's happening to Nanami, and asks Reina not to break up with Manabu so Nanami can have him like you'd expect from someone like her, but begs her for her help.
What snaps Nanami out of her funk — seeing Reina roughing up Sumika, she fights the former not because it's their usual routine, but in order to protect the latter — just like she used to do when they were younger. The flashback showed how much the two cared about each other, this chapter just served to cement it.
Shiratori finally mans up and gets the courage to tell Hijiri that he loves her...in his own special way, of course.
Reina chews out Manabu for not telling her about his feelings about his family's ugly situation...because she's his girlfriend and she wants to help him, because she wants him to be happy when he's with her.
The 'Shiratori' arc that spans a few chapters in the mid-30s. Manabu and Shiratori begin to understand each other and apologise for unwittingly upsetting one another. Shiratori apologises because he realises that Manabu grew up as a "Well Done, Son!" Guy child because his birth parents neglected him. Manabu apologises because he didn't think his non-recognition of Shiratori hurt him so much even though he was just being himself.
Ch. 66: Reina telling Manabu that no matter what, she will support him and do everything in her power to be with him. He starts crying and all the readers start wibbling.
Reina: If your uncle won't recognize me as your girlfriend, then I'll make him, no matter how many years it takes. I'll never give up. I'll find a way to make it work no matter what.
Once Kuroko passes on, Tsukihitsugi ends up having something of an identity crisis, not helped by the fact that she ends up being the only one who remembers Kuroko. Most of the cast, who have shown indifference to annoyance with her, try to cheer her up, not understanding the reason for her mood. Finally, it's the principal, who she's clashed with the most over her uniform and transformation hair, who is able to help her.
Principal: It is a "choice" that solves one's worries. You can choose to keep your beliefs and all they entail, or change them and take an easier path. You can regret your actions and adjust the way you've lived until now. And you can also follow what you believe in and opt to persevere onward. Always choose what you think is best, while being prepared to accept any pain that said choice brings with it. Now then, this is about as much as I can help you with. The rest is up to you.
The class trip arc.
Manabu and Shiratori's awkward bonding.
Reina packing her luggage full of fireworks and sparklers as to make up for the disappointing rainy festival and setting them off on the roof of the hotel.
Manabu telling Reina that he was choosing her over his family, finally at peace with his past and his guilt over possibly abandoning his mother.
The principal holding off on yelling at Reina for setting off fireworks on the roof of a building.
With encouragement from the principal, Kouichiro admits to his class that he struggles at talking to people and that he's going to try harder. Just the simple admission of this was enough to win his students over. Even better is that the next day they're shown interacting with him like any other teacher.
Reina going to the music room to see off Misaki/Kuroko...only to find that Aoi was there, waiting.
Note that before this, Reina had spent the day before pleading with him to come and see her off one last time. After being unsuccessful, the normally determined Reina actually gave up, because she realized that it's okay if Aoi decides not to come because at this point, nobody other than her and Tsukihitsugi remembers Kuroko's existence and because she would be just as unwilling to go if Manabu died and she was grieving. That's why it's such a surprise that Aoi still turned up. Thanks to how much of a jerk he's been throughout the arc it's easy forget that at the end of the day Aoi is really just a sad, angry man just lashing out at everyone because he's lost the person he loved the most, but certainly not in this instance.
It turns out the best way to get on Azami's good side is not to compliment her looks or demeanor. It's to speak well of Sousuke, her husband. For all that it appears their marriage started out as one of convenience, it's apparent that she loves and cares for him, in part to his stepping up to be a good husband, father, and Tanaka family member.
The final arc - The Ballroom/Brawl-room arc - is naturally going to be full of these.
When Reina is given carefully curated "evidence" that Tanaka might be cheating on her, she becomes upset... and shoves Prince Satou from her in sheer disgust. In most shoujo manga, the heroine would generally put the worst possible spin on matters. But this is Reina - who refuses to tolerate any of that bull.