This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Barefoot Gen
The Anime Movie. The whole goddamn thing.
Original Barefoot Gen drawings were donated by Nakazawa's wife to the Hiroshima Peace Memorial Museum after Nakazawa died.
Daikichi is devastated when his eldest son Koji announces that he's volunteering for the Navy, in order to get the neighborhood to let off on their persecution of his family. Enraged, fearing that his son is heading towards certain death or worse, Daikichi assaults him, reminding Koji about how his cousin Gokichi joined the Navy and ended up a blind quadruple-amputee because of the war. Later, during an air raid, Daikichi throws open the window and screams into the air for Koji to come back alive, tears flowing freely down his cheeks:
Daikichi: Fool!!! You fool. You're a fool, Koji!!! Koji, don't you ever die!! Whatever happens don't you die!!! Be a coward, a weakling... just come back alive!!!
While Daikichi reconciling with his son Koji and seeing him off by the railway tracks is a Crowning Moment of Heartwarming, it also is this, as it is the last time that father and son see each other. Koji revisits this moment later in the manga, when he returns from the Navy and learns that his father, brother and sister are dead.
"Father, are you out there? Eiko? Shinji? The baby's here, everybody! Can you see her? It's a girl! You were wrong Shinji! It's a girl! What's happening? Why? Why? Why? Why?! She's the prettiest girl in the whole wide world, and you never got to see her."
The mother picking up her newborn daughter and raising her in the air slowly showing her the destruction of the bomb. "Take a good look around little one. You see? This is the war that killed your father, remember it."
Also, when said baby girl dies, just as the two boys come back with food.
The live-action adaptation has the father and Shinji, buried in the weight of their crumbled house, spending their final moments singing on the former's urging. Most likely as an attempt by Daikichi to have his son succumb to the fumes, which would be more merciful than being roasted alive.
The soldier carrying Gen on his back to get medical attention, passing by a few wells with bloated corpses inside. The soldier tells Gen that they'd ran into the water, thinking it would save them from the fire.
That there is a boy and his little sister. He died trying to save her. And that woman there tried to protect her children by hugging them close to her. All in vain.
Most of the random civilians we see post-bombing are worth quite a few tears. The woman who thinks her son has reincarnated into a swarm of flies, the gang of orphan boys who all watched their own families die, the adolescent girl who wanted to be a dancer survives but with horrible burns on her face—she'd watched her mother die and walks among the corpses, checking their teeth for her mother's gold teeth, so she can bury her by her father...
Kimie's death by cancer due to radiation poisoning. As she's dying, she confesses she knew she didn't have much time left, but didn't want to let it show before her children (who also knew about it, but thought no one told her), so they would be strong. Then, after she passes away, Gen straps his mother's dead body to his back and plans to walk all the way to Tokyo just to show Gen. MacArthur and Hirohito what their war had done. Koji stops him before he can go anywhere past Kyoto (they had travelled there because it was their father's hometown, and Gen wanted to give his mother a nice memory before her time came).