In the final episode of Series 7, Thomas, one of the most reprehensible individuals in Hell, stands up for Scumspawn in front of God- because Scumspawn had tried to give Thomas a replacement for his lost dog, Scamp.
"Yes, well, you may have made the sun, and the stars, and the sky, and the volcanoes, and the continents, and the beasts, and humanity, and the myriad worlds, and the infinite wonders of the Universe- but he made me a CLOCKWORK DOG!!"
Actually, any time Thomas stands up for Scumspawn.
Scumspawn: "You saved someone who was being held at knife point by Jane Austen! How many people can say that?"
Also just about any time that Satan shows that he actually appreciates Scumspawn (that one time with the painting set and the saxophone comes to mind)
The Prof finally letting go of his (still living) wife and telling her to marry her new love instead of mourning him forever. Also, the end of the New Year's Special. The fact that there are so many Moments of Heartwarming in this show is proof of it's ultimately idealistic nature, despite being full of cynical humor.
Scumspawn's declaration of love is incredibly cute as well.
Scumspawn: "All the other demons serve you out of fear and terror, but I've served you and sacrificed myself to your will because I love you."
Satan: "I beg your pardon."
Scumspawn: "I love you. I don't fancy you. No, that would be physical and weird and probably quite dangerous. No, I love you in the widest possible emotional sense."
The ending of "Chuckles", when Satan, thinking he's finally broken the Professor, tells Scumspawn to tell the Professor that the version of Deborah he saw being so friendly to Dicky Freestone (a former colleague who stole his research) was really Scumspawn in disguise.
Scumspawn: Won't that lessen the torment?
Satan: There'll be plenty of other torments, Scumspawn.