This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / To Kill a Mockingbird
Jem feeling mighty awful that the hole that Boo has been leaving gifts and treats in has been plugged up by his brother. To Scout, it means the end of gifts while Jem has a bigger grasp on the cruelty toward Boo.
A good place to start would be "Stand up, Miss Jean Louise, your father's passing." Also counts as a Heartwarming Moment.
Another is "Hey, Boo." So much said in just two words.
And "Can you take me home?" from Boo to Scout. The idea that a grown man actually needs a little girl to take him from one house to the house next door because he can't go alone.
The ending when Scout stands on the Radley porch and sees the events of the book from Boo's point of view. There's just something so beautiful about that moment.
The fact that she never sees Boo again.
Older Scout:(narrating) I was to think of these days many times. Of Jem, and Dill, and Boo Radley, and Tom Robinson, and Atticus. He would be in Jem's room all night, and he would be there when Jem waked up in the morning.
"In the name of God, do your duty."
In the film, Gregory Peck's delivery of the line will send chills down your spine.
"I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do - seems that only children weep."
The film: Scout talking to Mr. Cunningham in the lynch mob, when the black community stood up as Atticus left the court room even though he lost the case, and when the reverend told Scout to "stand up, miss Jean Louise, your father's passin'", when Atticus receives the news that Tom Robinson has been killed and tries to convince himself the deputy sheriff didn't murder him, when Tom's father slouches into the house, moments after hearing his son is dead, being ordered around and called 'boy' by the man primarily responsible, and when Atticus, a man so fiercely protective of his children, trusts Boo Radley, a mute shut-in with a violent reputation, enough to leave him alone in a room with Scout and an unconscious Jem.
Atticus' speech about Mrs. Dubose, that horrible, racist old woman, calling her the bravest person he'd ever known for having the courage to die free of her morphine addiction.
"It just makes you sick, doesn't it?"
Raymond: Things haven't caught up with that little one's instinct yet. Let him get a little older and he won't get sick or cry...Cry about the simple hell people give other people—without even thinking. Cry about the hell white people give coloured folks, without even stopping to think that they're people, too.
The horrible, horrible realization that despite Atticus proving that Tom absolutely could not have raped Mayella—it was physically impossible for him, given his destroyed left arm, not to mention all but proving who actually did it—he's still found guilty of the crime. To the bigots of the town, justice and truth simply don't matter.
Jem's Innocence Lost after the trial. He grew up idolizing his father and the legal system, and can't believe the injustice. Scout notes it's one of the only times she saw her brother cry.
After the first day of the trial, the Finch family wakes up to find their back porch overflowing with food baskets prepared by the African-American community of the town to both thank Atticus and serve as a form of payment for his helping Tom. It's beautiful, but also somehow tragic—this was all they could do to express their gratitude, and they can't even do it in daylight.
The fact that Mayella can mistake basic courtesy as Atticus "making fun of her." Likewise, the fleeting implication that her father sexually molests her (though this is absent in the film).
The image of Gregory Peck holding Mary Badham (Scout) on the seat on the verandah.
The scene where Atticus sits on the porch and listens to Jem and Scout talk about their late mother.