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.
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.
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!