This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / The Breakfast Club
Andy's monologue about how he regrets bullying a weaker student, and how his dad pushing him so hard to succeed at wrestling has made him not even enjoy the sport anymore. Even Bender can't look anyone in the eye during his speech because it's so sad.
Pretty much the entirety of the scene once the kids have settled down following the scene with the pot; They vary from being at each other's throats to genuinely caring for one another, but what struck me the most was this moment; the compulsive liar◊ of the group basically opens up her heart in the sweetest of ways.
After Allison says, "When you grow up, your heart dies." John, stung but trying to act cool, flippantly asks, "Who cares?" Alison's tearful reply, "I care", gets me every time.
In the same scene, Claire's tearful declaration that she will never be like her parents.
The part where you realise that Allison is absolutely telling the truth when she claims her parents ignore her; Watch the beginning of the film again, and see the scene where her parents just dump her and drive off without so much as a word.
The end of the movie. Lots of romance, lots of happiness and optimism as they leave the school. And the odds are good that as soon as they get to school at 9:00am Monday they'll pretend not to know each other, or at least Andy, Claire and Bender will ignore each other and Brian and Allison.
The scene where Bender is acting out his father's mannerisms. The amount of detail and the way his voice gets louder with every word and movement is heartbreaking.
When Vernon shoves Bender in the closet and actually physically intimidates him. It's one of the few times we see Bender genuinely vulnerable. His small, scared, "You're threatening me?" takes the cake.