The scene at Theresa's dinner table, between Juan and Chiron, during the 'Little' segment. Just watching Juan's face and silent nods and feeling the silence grow stronger, as he answers Chiron's Armor-Piercing Question sequence. It showcases why Mahershala Ali deserved his Oscar, but it also breaks your heart.
Little: You sell drugs, right?
Little: My momma...she do drugs, right?
Juan: (bowed shamefully)....Yeah.
(Little slowly gets up, and leaves.)
Seeing the relationship between Paula and Chiron deteriorate, particularly in Act II, where he is witness to what his mom can be like when she's off her drugs. Culminating in her reconciliation conversation with him at the end.
Paula: You ain't got to love me, but you gonna know that I love you.
Terrel manipulates Kevin into a hazing ritual where Kevin has to punch Chiron until the latter can't stand. All the while, Kevin looks increasingly distraught and pleads with Chiron to lay down.
When speaking with a social worker, Chiron breaks down and refuses to name his attackers as he believes it won't change anything.
The next day, Chiron snaps and he assaults Terrel in class, leading to his arrest. As Chiron is taken away by the police, Kevin guiltily looks on while Chiron glares back. It's heavily implied that their relationship fractured and they lost all contact until the events of the third part of the movie.
When Chiron and Kevin reunite as adults in the diner, Kevin is excited to see Chiron and eager to hear about his life. When Chiron finally admits that he's become a drug dealer, the light instantly leaves Kevin's eyes and it takes him a moment to muster just a Flat "What".
The final shot of Kevin hugging Chiron, after Chiron admits he still loves him.
Chiron: You're the only man who's ever touched me.