Harry's final conversation with Lasciel, where he makes her realize that she does have a choice and that she can redeem herself. And then, in the final confrontation, she accepts the psychic backlash from escaping the mind-control spell, effectively sacrificing her existence for the sake of Harry's. Redemption is, indeed, a beautiful thing.
Minor clarification: It wasn't Lasciel the Fallen, but Lash, the "shadow" living in Harry's head. Harry actually gave her a portion of his soul, and her sense of self in the process, which just makes it more heartwarming.
Even more heartwarming when you consider a certain revelation in Skin Game: the fact that this moment was such a potent act of selfless love that it gave birth to a spirit of intellect made of both Lash and Harry's souls.
The final conversation of the book between Ramirez and Harry, with Ramirez saying he's with Harry 100%, especially when Harry watches over him sleeping the whole night because "you don't leave an injured friend alone."
Molly, terrified and more than a little scarred, finally learns why Harry fights, and still steps forward to fight as well. Her father's daughter, indeed.
Glimpses of Marcone's inner good nature always end up being CMoHs. The best is in here, when Marcone is providing an extraction route for Harry and the battle gets heated. Marcone tells Harry he didn't sign on for a war. Harry responds by going up to him and saying:
"People are dying, John. Help me save them. God, please, help me."
"Who do you think I am, wizard?"
"Someone who can help them. Maybe the only one."
He stared at me with empty, opaque eyes. Then he said very quietly, "Yes."
Murphy sits Harry down, and has a calm talk with him outlining her worries about his growing anger issues and Harry (eventually) listens and admits she had a point. Considering where these two characters started, it really emphasised the trust and friendship that has grown between them.