The episode "911", especially Dee-Dee watching the EBS message with rapt attention, reacting throughout the episode as if she were watching a comedy, a horror movie, and a love story.
"The Koos Is Loose": "If you turn your lips inside out, you look like Jimmy Carter."
The entirety of "sdrawkcaB", in which Dexter invents a belt that allows him to go in reverse. It leads to him talking backwards, un-eating a sandwich, running up and down the stairs, and then discovering that Dee Dee has been messing with the forward/reverse switch: "*gasp* Dee Dee!" *switch* "!eeD eeD *psag*". He finally one-ups her when she puts on the belt, letting her fall to the floor from a great height, then flipping it into reverse several times.
Towards the end of the episode, "The Continuum of Cartoon Fools", Dexter tries various ways to find out how Dee-Dee got into his lab, and in this scene, Dexter pretty much goes bonkers by smashing a watermelon.
The entire episode of "My Dad VS Your Dad". "ARE YOU CRAZY?! YOU JUST THREW A DUCK AT ME!"
"Monstrosi-Dee Dee" is a CMOF. Funniest part being Dee-Dee giving Dexter various hiding places only for her as a monster to discover them and Dexter practically failing to realize that the monster is his sister.
In "Jeepers Creepers, Where Is Peepers," Dexter and Koos' annoyed response to the Animesque villain Hukuchu's drawn-out monologue:
In particular, the "Oh s***," ending and Polite!Dexter and Dee-Dee.
The ending of "Things That Go Bonk in the Night", when it turns out to be All Just a Dream, Puppet Pal Mitch's dream to be exact. They resolve never to stay up and watch the four-day Justice Friends marathon again, and wish each other good night. Then:
Puppet Pal Mitch: Puppet Pal Clem! There's a spider in your bed!
*Puppet Pal Clem sits up and hits his head on the top bunk bed*
Puppet Pal Mitch: BONK!
"Dimwit Dexter", starting from the moment he turns stupid to the end.
The snowball fight episode brings us this scene.
Dexter: First base... (Pan up to show Dexter his hiding behind the legs of a couple making out) Kissing dude: You said it, brother! (Cue Rimshot).