This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Ender's Game
Ender's Game: "The most noble title any child can bear is third", " I didn't want to kill anyone — why wouldn't they leave me alone?", " He fought with honor. I didn't fight with honor. I fought to win.", "Do they know me well enough to know I don't fear death?"
Ender: In the moment when I truly understand my enemy, understand him well enough to defeat him, then in that very moment I also love him. I think it's impossible to really understand somebody, what they want, what they believe, and not love them the way they love themselves. And then, in that very moment when I love them... I destroy them. I make it impossible for them to ever hurt me again. I grind them and grind them until they don't exist.
And in Ender's Shadow, when Bean, alone among Ender's jeesh, realizes that the "simulation" they're playing is real and they are sending real men and women to their deaths, and laments for them: "O my son Absalom. My son, my son Absalom. Would God I could die for thee, O Absalom, my son. My sons!"
Also in Ender's Shadow, at the very end, when Bean finally gets to meet his parents, and his mother, who has just become aware of his existence around 10 seconds ago, opens her arms wide and shouts "Here are my sons, back from the war! I'm so proud of both of you!" or something to that effect. This causes even Bean to burst into tears; this is the kid who showed barely any emotion through the entire book.
And shortly before that, in the dialogue between Graff and Sister Carlotta at the start of the last chapter when Carlotta tries telling Graff how he saved the world.
"Children, Sister Carlotta, the things I did to these children."
"You gave them a world to come home to."
"All but one of them."
At the end of Shadow of the Giant, Bean says goodbye in a letter to his wife, Petra, knowing he'll never see her again. The saddest part, though, is that their friend Peter has been reading the letter to her, but when she reads it, she realizes Peter added "I love you" to the end.
From the movie, when Ender is confronting the Hive Queen just inches from her egg. She reaches a claw out to, presumably, kill Ender to avenge her species. But she stops, and withdraws her claw, gently stroking his face instead. The single line that Ender speaks to the Queen clinches it. May also double as a Crowning Moment of Heartwarming.
Ender: "You're dying..."
"Mother. Father. Did I do it right?"
By the end of "Ender's Game," Ender believes his parents had forgotten him. In a short piece by Card, we find this isn't true and that Ender's parents, who are still very much pining for their son, took some measure to keep Ender off Earth to protect him.
From Shadow of the Giant, Graff's final letter to Bean. Again, doubles as a Crowning Moment of Heartwarming.