Dexter's dad. When he gets serious, he gets so goddamn EPIC.
Special mention goes to the career day episode. To sum it up, Dexter's school is having a career day and all the other kids are talking about how cool their parents' jobs are. Dexter has no idea what his dad does, so he tells everyone that his dad is a government scientist, which the other kids think is cool. Dexter creates a fake dad in his lab, but when he actually brings this dad to school, Dexter's classmates don't like him. Dexter's real dad (whom Dexter didn't tell about the career day, but he knew about it from a memo in the mail) sees that Dexter's getting a very poor reception. Dad finally reveals what his job is: a professional motorcycle stuntman.
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, and then starts to literally swab the deck with them.
Dee Dee calling sea sponges to turn Wet Willie into dust.
The entire sea battle, really. Features giant crab-like mechs, cannon-loaded ships, dolphins, whales, explosions, etc.
Monkey impressing a pro-wrestling alien named Rasslor by never giving up, even after Rasslor had destroyed the rest of the Justice Friends and refused to fight Monkey, thinking he was too weak. Also doubles as a Crowning Moment of Heartwarming.
Rasslor: Incredible. I could crush your body, I could smash your bones, but I could NEVER break your spirit.
Let's not downplay the fact that Rasslor was voiced by Macho Man Randy Savage. Go ahead, re-read the above line in his voice.
The entirety of "Last But Not Beast", while it certainly is a spoof of the anime genre and Kaiju films. It was still really cool watching Dexter and his entire family being totally kick-ass awesome!
Watch Earth's forces kick ass was awesome as well. Everyone from Agent Honeydew's air force to the Justice League to the Commander himself in a little TV that fires missiles to Monkey. Even that little foreign exchange boy got a moment of awesome.
In "Accent-U-Hate", when Gary the bully decided he was going to beat up Dexter and his friends because they "had funny accents," just about every other kid in school showed up and ordered Gary to leave them alone.
Dexter's epic speech to his future self, convincing him to get out of his wimpy rut and come with him.
Dexter: Stop the typing. Stop the typing. [crushes Wimp!Dexter's hands with his boots] STOP THE TYPING! You are not a worker drone, my friend! You are not just a number! YOU ARE A GENIUS! [slams his chair against the wall] And YOU'VE got a name! What is your name?
Dexter: NO! What is your name?
Dexter: NO! What is your NAME!?
Dexter: Yes... yes...!
Wimp!Dexter: DEXTER! [laughs with joy]
Dexter: YES! Dexter is the name of a LION, a TIGER, a BEAR!
Wimp!Dexter: Oh my!
Dexter: Now let's get the heck into the future and see how cool we are!