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's last line, though it borders on Tear Jerker.
Superman: I promise you, I'll never forget.
For clarification, this line is heartwarming because even though Superman's son was not real, the memories of watching him grow up were as real as any other. And even if Van-El did not actually exist, he simply can't bring himself to dismiss the memories of his beloved child.
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.
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 Bruce is actually showing it. He trusts Diana (and Clark too, no doubt) enough to drop the cold, brooding act and just be himself. There's very few people on the entire planet who can say that.
For this troper, seeing Diana lose her temper at hearing Mongul call Superman's 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.