The episodes that show us Rei's broken psyche, as well as her death and everyone's reactions to it. She was finally getting a chance to be happy, and in the way to what could've been her and Nanako's first official date... she gets hit by a train. Even Fukiko, who treated her so badly, collapses in tears. Not to mention, there's also poor Nanako being borderline cathatonic for days, her talk with Kaoru in which for the first time ever Kaoru can't help her, Rei's fangirls crying during the wake, Fukiko's last goodbye as Takashi desperately tries to keep people out for some minutes so Fukiko make peace with Rei, even if it's only in front of her lifeless body (finishing with a Last Kiss); Nanako smoking for the first time to give her respects to Rei (the cigarette was one of hers), and her heart-to-heart with Fukiko in which she receives Rei's porcelain doll as a Tragic Keepsake.
In the manga, Kaoru's death, especially right after seeing her happily flying off to Germany with Takehiko.
Not to mention her almost crossing the Despair Event Horizon right after Rei's death, coinciding with her thinking she'll have a relapse in her breast cancer. (Which in the manga turns out to be trye)
"Mona Lisa" Komabayashi's Not So Stoic moment in the Sorority dissolution arc, when she realizes how far she and "Borgia" Ogiwara have gone, saves Nanako from falling off a balcony, and then breaks down in tears and begs Ogiwara to stop.