In The Horrendous Space Kablooie: After having had his beloved bowler hat destroyed in The Promethium Closes, Brooke Rhapsody uses a matter-alteration setting on her gun to create a new one in midair. The look of thanks on the Inspector's face when Angie catches it and plops it on his head is heartmelting.
Inspector Spacetime: Why did you do it Reginald? You sacrificed your name, what you stood for. Even what's left of your poor broken heart to save me.
Constable Reginald: Somebody had to do it, Inspector. You're the boss and I am the loyal sidekick. And the cost was... acceptable.
Inspector Spacetime: (somberly) Surely, I am not that good of a boss.
Constable Reginald: No. (beat) But you're that good of a friend. Besides you would have done the same for me.
Inspector Spacetime: -Without a second thought or a moment's hesitation, old friend, and the Universe herself could not stop me.
Along with being quite a Tear Jerker, Lily's ultimate choice to leave this physical universe and embrace her destiny as the Good Lamb by joining the Pantheon of Immortals counts as a Heartwarming Moment just for the sheer selflessness of the act. Her goodbye to the Inspector is adorable, too.
Lily Taylor: You are my shepherd, Inspector, and I am yours. Watch over our flock for me.
(Lily shines with the energy of the Good Lamb, and ascends into the heavens. The Inspector smiles, proud and yet regretful at his associate leaving. We linger on one final shot of Lily, clad in the Good Lamb's white robes and carrying a shepherd's crook, as she enters the Pantheon of the Immortals. Roll credits.)