Cat completely losing it and destroying everyone in "Monster Truck Folly".
The end of "Brother's Day." Winslow tried to set up a Gift of the Magi Plot, with Cat selling his luffa sponge to buy Dog a mud-wrestling costume and Dog selling the mud from his ring. However, instead of the luffa carrying case, Dog bought Cat a mud-wrestling costume, too. After Winslow expresses disappointment over this turn of events, Cat and Dog don their costumes and tag-team him as payback.
Dog: Looks like it's curtains for the little blue guy!
Cat's prank in "Kooky Prank Day," which is so complex (involving an alien invasion and robots) that to pull it off perfectly is awesome in and of itself.
Dog defeating the antagonistic, Smug Snake mailman who taunted him and called him a slowpoke.