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.
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.