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." - Batman pleads with his (dead) parents and himself to let himself go from the path of vengeance of vigilantism in exchange for happiness with Andrea.
The flashback, in which Bruce receives the "Dear John" letter.
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 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 comics, it might have been too late. Bruce calls Andrea a "murderer", and Andrea doesn't seem to care anymore.
Bruce's despair at the end and Alfred's speech also counts:
Alfred: I don't think she wanted to be saved, sir. Vengeance blackens the soul, Bruce. I've always feared that you would become that which you fought against. You walk the edge of that abyss every night, but you haven't fallen in and I thank heaven for that. But Andrea fell into that pit years ago, and no one, not even you, could have pulled her back.
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.