One hilarious, if painful for Timmy and Jimmy, was when Jimmy gives their creation the name "Shirley" and assures him that it's a cool name. When "Shirley states "Beware the wrath of SHIRLEY", Timmy, not known for having very much self-control anyway, bursts out laughing saying "That's the dumbest name ever." Shirley angrily says to Jimmy "You gave me the dumbest name ever?". The look on Jimmy's face is PRICELESS!
When they are creating the villain, Timmy says they should give him a moustache because "moustaches are evil". Cosmo tells Timmy that not everyone who has a moustache is evil. Wanda says "Your mother has a moustache", which causes Cosmo to relent.
After Jimmy loses his genius, Timmy and Jimmy end up using useless inventions from Jimmy's lab. Timmy yells reactively "Cosmo, you idiot" before apologizing due to habit and correcting "Jimmy, you idiot."
From the first one, Crocker is sitting in a locker, about to press the button to go to the Crocker Cave, when Principal Waxelplax walks up and sees him. They look at each other for a few seconds, and Crocker screams "FAIRIES!"
The entire "Breaking Da Rulez" scene in When Nerds Collide is one long sequence of these. First, Carl falls in love with a turkey, then Cosmo uses magic to constantly beat Sheen at Rock-Paper-Sissors while both of them go to Broken Record levels to remind Jorgen that he's using magic to do this, and finally, Jimmy wishes for Timmy to be dead, leading to him "dying" in a ridiculously hammy fashion and Cindy wailing over him in equally hammy grief. Jorgen's increasingly infuriated reactions make it better.
"I'm riding a hot dog over you! All these pork byproducts have got me feeling blue!"