This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Brightest Day
While readers may know Martian Manhunter was accidentally summoned by a scientist in his origin, few may not know that this actually happened during the era of the Golden Age heroes. So when J'onn goes to meet with Professor Erdel's daughter, she is very old... and becoming senile. The whole scene makes you want to cry and J'onn's face at the end truly says it all...
Booster Gold learning that Maxwell Lord's mind wipe made it so that everybody thinks Ted Kord committed suicide, instead of being murdered by Maxwell Lord.
The origin of Dex-Starr , Atrocitus' cat, as revealed in Brightest Day.
That last panel was enough to make a cynical comics reader cry.
The origin of Saint Walker "All will be well", Atrocitus "They came, and we... and we did nothing...", and Sinestro. "Was it worth losing the only woman you ever loved?" To make matters worse, recent issues are dropping hints that even Larfleeze of all people, has a tragic past.
Captain Atom flying the Rocket Red Max Lord booby-trapped away from the rest of the JLI to protect them, all while both the JLI's new Rocket Red and the aforementioned RR are begging for him to help.
Ivan: You help me, da?! Captain Atom?! You are going to stop this?! Yes! * beat* Captain...?
Captain Atom: Be brave, soldier. Be brave.
Jade finally coming home and just when she gets back to her family she's told she and her brother can't be anywhere near each other for fear that they'll involuntarily fuse together again. Particularly heartbreaking as the twins have always been close, even before their father got to know them.
The last page of the Larfleeze Christmas Special, where we learn that what Larfleeze really wants for Christmas is his family.
After millenium of trying and finally breaking their curse, Hawkgirl and Hawkman's new, and now last life, is silenced when the White Ring KILLS THEM, when they refused to be a part of the ring's plan.
After the White Ring returns those it had earlier killed and turned into elementals to life, it leaves out Hawkgirl. Hawkman is not happy.
John Stewart's Shoot the Dog moment in killing Mogo to prevent rings from recruiting new Lanterns to be brainwashed. Soon followed by Hal's firing from the Green Lantern Corps by the Guardians.
Boston Brand getting killed and turned into Deadman again, and his farewell to Dove.