When Dexter learns that pirates kidnapped his sister (they believed she was a mermaid due to her swimsuit), he goes straight to the pirate ship and reveals his Giant Mecha, which he had intended to keep secret, to save his sister, even though it would be easier for him if she were out of the picture.
In a weird way, the ending to "Beard to Be Feared", even though it's had some Memetic Mutation:
Action Hank: "It doesn't matter if you have the beard on the outside, as long as you got the beard on the inside."
The first half of "LABretto" is really nice and funny, and then the rest is just funny.
The ending to "Dee-Dee's Tail". "Run free, Dee-Dee!"
Made even better by the fact that this one of the few genuinely happy endings in the series.