The title sequence with the monstrous giant attacking the city. Unlike the others, this one is a well paced sequence that has a menace worthy of Underdog's power as a Flying Brick.
Part 1 of "Fearo" has Underdog fighting off a sea monster while still in his Shoeshine Boy alter ego (everyone else is hiding from it, so he could fight it without risk of revealing his secret identity). Considering Shoeshine Boy is usually treated as The Chew Toy by most antagonists, it's cool to see him get to kick ass without having to switch to Underdog for once.
Underdog finally breaking free from Simon's control in "The Phoney Booths, part 4". When Polly tries to destroy the booths with a fire axe, Simon sends Underdog to destroy her, but Underdog fights from the inside and breaks his control.
Underdog stopping Riff Raff's scheme in spite of the former's back pain in "Pain Strikes Underdog".
Even with amnesia, Underdog still saves people in "The Forget-Me-Net, part 3".
Underdog helping the pilgrims with their requests in "Simon Says, 'No Thanksgiving!'", and later fixing the fort and tepees after the fire is out.
Underdog catching all the jars containing every body of water and releasing some of it to wash away Simon and Cad in "The Big Dipper, part 4".
Underdog beating the whole group of bad guys in "The Vacuum Gun, part 4".
Polly helps too by diving for his ring when it falls underwater. She initially fails at finding the ring, but Underdog inspires her to not give up.
The live-action film
The remix of the classic theme (strangely not included on the soundtrack, thus a proper extended version isn't available), especially effective during the montage showing Underdog at work and at play. "So you're a cat burglar, huh? Well, I'm not really a cat person. Woof." Cue said burglar being slammed into the hood of a cop car.
The climax, where Underdog thwarts Barsinister's mind-control plot (after regaining his powers and convincing Simon's powered-up dog mooks to stop working for him due to his mistreatment of them): he buries the bomb in the ground, and while the explosion still happens, it's enough to hurl Underdog into space. He's thought to be dead when he lands, but he managed to survive.
, is filled with wonder, even as a meme.