This moment at the end of series 3. Sally, having been panicking over her recent declaration of love for Patrick, is told by him how great she is (which she had been discussing before with her friends as the worst kind of rejection), and he tells her, "I wanna see you with the best. You need Mister Amazing. Mister Incredibly Superbly Fantastic... ness. In your heart, I'm sure you know I'm right." At which point:
Sally: I don't want Mister Superbly Incredibly Fantasticness, you stupid, stupid ass. I want you. Patrick: For God's sake, Sally... Sally: ... What? What?! Patrick: I wastalking aboutME!
Followed by THE SPIDER-MAN DANCE. Easily the most romantic, heartwarming use of the Spider-Man theme music in televised history.
The final scene of the programme, when Susan had the baby, and Steve just stares at it, confused that he's not feeling anything warm or paternal towards it. Then he notices that the light is very bright and "So I shaded his eyes. And then... and then he looked at me. And oh my goodness me. I became someone else entirely."
Jeff and Julia finally getting together, after the accidental strip tease, was both this and a CMOA. Made even better by Jeff's childish joy at having an actual, real life girlfriend.
Steve, in the middle of a truly remarkable rant about how natural it is for men to like porn, blurting out that he wants to spend the rest of his life with Susan. Knowing the backstory, it's basically a public declaration of love from Steven Moffat to his wife Sue Vertue.