For an in-show example, how about Wilt's Cluster Big "NO!" at the end of "Where There's A Wilt There's A Way?"
In Bloooooooo, Frankie DRIVES THE BUS THROUGH THE (LOCKED) FRONT DOOR.
Eduardo rescuing all of the friends from the Pikachu-ripoff creature. Someone that hysterical managing to stay calm, cool, and collected for that long deserves some kind of medal.
Him finally standing up to Terrence also counts.
"I AM NOT A SCAREDY-BABY!!!!!"
Bloo in the episode "I Only Have Surprise For You", when he arranged a Xanatos Gambit to humiliate his friend Mac while celebrating his birthday, a move that just made him more of a Jerkass to some, but proved that a character often portrayed as having the IQ of a walnut actually has some genius tendencies, when he's not being a Cloud Cuckoolander.
IN "Duchess of Wails", Duchess finally gets adopted. However, the family that adopts her lives next door to Mac, and because she won't stop yelling and screaming, Mac's mom plans on moving away. Mac and Bloo try to get Duchess back into Foster's, only for Madame Foster to act as a general in keeping her out. What ensues is a full-out war of everybody in Foster's vs. Mac and Bloo, where the members of the house use produce as artillery. The entire scene is epic, but Madame Foster's speech before the battle is just fantastic.
Madame Foster: Yes, my imaginary friends, these three will be devoured by Foster's like bagels with lox! I say we shall outlast them all! We shall make this night one worthy of remembrance! Alright my imaginary friends, TO WAR!
Mac: What!? How did they know we were coming?
Bloo: The mighty eye of Foster sees all.
In "Frankie, My Dear," Frankie's Unwanted Harem makes her date extremely difficult. When she goes to the ladies' room, the date reveals himself to be a Jerk Ass, and insults her harem (Bloo and Mac among them)... without realizing Frankie is right there. She proceeds to kick his ass. And then they watch the whole thing on public news later. With popcorn.
Madame Foster attempting to protect the group from a bear in the episode "Camp Keep A Good Mac Down".
Mac winning the race at the end of "Race for Your Life, Mac and Bloo", even though if he ends up in the hospital for exhaustion and dehydration.