This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Batman: Mask of the Phantasm
"I didn't count on being happy."
"I didn't count on being happy." Bruce pleads with his dead parents and himself to let himself go from the path of vigilantism in exchange for happiness with Andrea.
The kicker? There have been subtle hints during the show that Bruce's parents adored him and would've wanted him to be happy.
The flashback in which Bruce receives the "Dear Bruce" letter from Andrea.
Alfred's shocked and despairing "My God!" when he first sees Bruce in full costume after Andrea leaves him.
Even worse is the moment right before. Bruce looks at the cowl, the last piece to put on, and stares at it. It's his one last chance to back away from his vow and start over. He put it on and never looked back.
The shock and horror in Andrea's voice when she sees pre-transformation Joker leaving her house and realizing that he just killed her father. Not to mention the bloodcurdling scream she lets out when she sees the body.
The last two lines, which sum things up for both Batman and Andrea. "Do you wanna be alone?" "I am."
What makes it worse is that, in the end, Bruce would have taken Andrea back despite what she's done. Yet, when they meet again in the tie-in comics, it might have been too late. Bruce calls Andrea a murderer and Andrea doesn't seem to care anymore.
Realizing that Andrea may quite possibly be the only person who could understand Bruce completely and totally yet, unlike Bruce, lost herself to her desire to punish criminals long ago, something that Bruce regularly fights against within himself every time he puts on the cowl.
Since he can't ever catch his parents' murderer, this case will probably be Bruce's most personal. Though he's The Stoic, there's a lot of suggestion that digging up these memories and finding out these terrible truths is leaving him absolutely broken.
Joker: What are you doing?! Let me go or we'll both die!