Calvin and Retro's confrontation in "Our Solemn Hour Part 2" definitely qualifies as this.
As well as Calvin's "little" plan to defeat Rupert once and for all. It's amazing how much time he took to think it all up!
Dr. Brainstorm's Let's Get Dangerous! moments inevitably all involve someone's ass getting kicked (Thunderstorm being the main victim of this).
While Evil Jack wasn't original at all, he did have some pretty awesome features that even Dr. Brainstorm didn't know he had.
Just what DID happen to the bathroom in "POV"? It had to have been really bad to have everyone in the main cast mention it in their stories.
Elliot in general, in a meta source of way: he has avoided showing his face and hasn't even said one word of dialogue until the end of Season 4! He even managed to keep his very name hidden until mid-Season 2!
While seeing Socrates go completely enraged in "Have You Seen This Tiger?" was heartbreaking, it was pretty awesome seeing him kick Calvin's ass mercilessly (although the jerk deserved it!) and managed to give Hobbes a Humiliation Conga that even made him lose his respect.
"Thunderstorm" has a lot of this, be it Dr. Brainstorm's aforementioned Let's Get Dangerous! moment, Hobbes finally growing a backbone, or the very last fight scene.
"Nocturnals" has Calvin giving a speech when told he's Just a Kid:
"I may be a child. I may go to school and do homework and get beaten up by bullies just like other kids, but it just so happens I'm a child who knows how to get us out of this mess and save the world from an otherworldly creature that you two know nothing about, and don't pretend that you know what you're doing because you're older than me, because I know you're both lost and just trying to be important, so get your heads out of your nether regions and listen to me, okay?"
This rather impressive bit of writing from "Invasion", as the protagonists (sans Calvin) find themselves running through Annkor:
Hobbes was in front, struggling to lead the others to a place of safety. Calvin wasn't here, so for the time being, he had to give up his cowardice. Calvin had once referred to him as the Earth Potentate's Second-in-Command. It was time to live up to his false title. Hobbes could only force out his bravery and lead.
Andy was running behind him with Sherman safely huddled in his pocket. He was quite terrified. He tended to get a little scared in these experiences. When he thought about it, his life had become incredibly complicated. Sure, it had been strange before. He had a talking pet hamster for pity's sake! And yet he'd been content with this life. Now he was on an alien planet dodging energy blasts. It both amazed and terrified him. There was a world beyond his videogames.
Sherman was tucked away in his pocket, poking his head out as he bucked up and down with Andy's right leg. He was digging his small claws into the blue jeans. This was the price he paid for being so intellectually curious. He was a scientist, after all. Sure, his experiments on Earth were a bit frivolous, but he enjoyed them. Now, however, he was admiring the bigger picture and the intelligent life that was before him.
Socrates, for once in his life, was not complaining about not pranking someone. For once, pranking was not on his current docket. He was concerned about getting around the alien planet and surviving. Helping this alien planet would become a priority as soon as his life wasn't in danger. And seeing as how he was at the rear, odds were that he was going to be in danger longer than the others.