For this troper, its when Superman runs off and hides his replica of Kandor, so Wonder Woman doesn't feel bad about giving him a gift he already had. He does this so fast that nobody can tell he stopped talking and left the room.
From the DCAU version, Supes last line, though it borders on Tear Jerker.
Superman: I promise you, I'll never forget.
Batman finally trying to talk Clark back to reality when all of his attempts to remove the plant through the use of science and tools fail.
Batman: [Wonder Woman]'s fighting for her life, Clark. You've got to fight too.
The especially heartwarming scene is Diana, who has been fighting Mongul and is badly injured, trying her absolute hardest to get to Bruce to remove the Black Mercy. She literally crawls towards him. That is an unparalleled love and friendship between them.
Hell, just the premise of the episode is heartwarming. Batman, the scariest vigilante on the planet, is bringing his friend a birthday present. What's more is that he feels awkward about it because he didn't know what to buy. Try to wrap your head around the fact that Batman felt socially awkward. And then there's Diana teasing him while they're on their way to the Fortress of Solitude. The first few minutes of the episode are adorable.
Doubly so given that this Bruce is actually showing it. He trusts Diana (And undoubtedly would have been the same around Clark) enough to drop the cold, brooding act and just be himself. Given who were talking about, there's likely only one other person on the planet he'd do that for.
For this troper, seeing Diana lose her temper at hearing Mongul call Supermans heroism an act. Hearing him being a sexist prick was bad enough, but she held her ground. Insult the nature of one of her best friends and she will mess you up.
In the comic, Batman's Black Mercy fantasy has him marrying Kathy Kanenote The Pre-Crisis Batwoman and having a teenage daughter.