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.
Dexter's mom's badass catfight in "A Mom Cartoon".
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 Friends 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. Gary ends up so disoriented by their yelling he smashes facefirst into the school statue, giving him a "funny accent" from his swollen jaw.
"Dee Dee and the Man" where he fired her as his sister was pretty satisfying for everyone tired of Dee Dee's crap.
Dexter using a Mini-Mecha to win at dodgeball. Especially since the school bullies had spent the entire previous week torturing him during dodgeball. Dexter proceeds to beat the crap out of them, even using dynamite against one of them hiding in the jungle gym.
The episode "D & DD", from the opening sequence to Deedee's part of the story.
The fact that Dexters friends has way more fun playing with Dee Dee as the DM, because Dexter just went out of his way to kill them.
My Dad vs. Your Dad has this with both Dexter's and Mandark's dads fighting one another.
The episode "Reparanoid". Mom notices that the wiring in the house is acting up and calls him an electrician named Arnie. While fiddling around with the wires, Arnie finds one that doesnt seem to be hooked into the rest of the house, and follows it, only to stumble across Dexters lab. His reaction to seeing a gigantic lab full of advanced technology? He hikes up his pants and gets to work right away. Dexter freaks out when he finds out someone has discovered his lab, only to realize that Arnie has managed to rewire one of his machines to work at 100% efficiency, something not even he has managed to do. Arnie proceeds to bring the whole Lab up to code, including robots and all manner of computers and devices no normal electrician would usually work with. Dexter even shakes his hand in gratitude afterwards.
The incredible Musical EpisodeLabretto, which is presented as a stage opera about Dexter and Dee Dee's early childhood. For example, there is Dad being shown his newborn son after much anguish.
Dad: My goodness, my gracious, when will this day be done, will it be a daughter or will I have a son?!
Doctor: Congratulations Mr Father, today is finally done! May I announce that now yoouuu haaaave a soooon!!*the group of doctors part, revealing Mom with an infant Dexter in her lap*
While jubilant over his son, Dad is shocked at the size of Dexters head, and asks if there's a problem with him. The doctor responds with this:
Doctor: He proves to have a skull exactly twice the size his norm. And Norm's the largest man I've seen and you have just been born! Our X-ray's shows his brain is huge, it shouldnt be a fuss. As doctors we must classify him - CLINICALLY GENIUS!! *is handed Dexters trademark glasses on a silk pillow* With these glasses I bestow to you, the knowledge and skill that is quite far and few!
Dexter: Atlast Mother and Father, I am created!
Dexter's second words ever, quite apt for a genius.
Dexter: For my first observation I would like to state; for every action there is a reaction!
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!