Po taking down the amnesiac Furious Five in "Fluttering Finger Mindslip"... with a chair.
Fridge Brilliance ensues when you realise that these Furious Five were actually trying to hurt Po, unlike the usual case where it's training.
Po revealing how truly awesome his kung fu is in "Hometown Hero", when he goes off into the forest to leave the ungrateful Mantis in the lurch.
Po and Tigress learning to work together while chained to take out the Croc Bandits in "Chain Reaction".
Shifu showing up in disguise as the Croc Bandit who has caught the Sacred Hammer in "Jailhouse Panda".
Po not only fooling Fenghuang into thinking he was a Fake Defector, but using the toyknife on Shifu so as to get her close enough to be locked in her prison.
Po's switcheroo trick to get his Evil Twin to look in the mirror.
Prior to that, Good Po taking advantage of their Synchronization to beat the crap out of himself. As well as their last exchange:
Bad!Po: (enraged) No! This Po does what he wants!
Good!Po: (serenely) And this Po does what he needs to.
In a moment of pure Papa Wolf, Ping pulls off a difficult kung fu move to protect his son, Po.
Not only that, but when the Furious Five refuse to help Po take back the jade Palace in "Sight For Sore Eyes", Ping calls them out for it. When Tigress coldly refuses, the following reaction follows
Ping: (Outraged) Would you MIND taking off those ears?!?
Crane: He probably just made that up about Shifu. I mean, Junji is a shaolin master!
Ping: (Grabs Crane's beak and pulls him down) Listen to me, limb-beak! My son may be not perfect! He may be big and loud and clumsy and annoying and impatient and hot-headed and...where...where am I going with this?
Oogway's ghost laying a smackdown on Junjie. Even if it was actually Shifu in disguise.
Peng. Just Peng, throughout pretty much his whole introductory episode. A wide-eyed little snow leopard who takes down a much larger opponent as soon as we meet him, goes through the whole training hall with such skill that even the Five are left stunned and speechless for a few moments, and even goes toe to toe with both Temutai and Po. All of this without any type of formal training. It all makes sense in the end of the episode, though...
The fact that despite it being In the Blood for him to do so, he does not rampage, but calms down and apologizes once Po admits his lie.
In "My Favorite Yao", how Shifu uses his fan dance to get himself free of his cuffs, and then how he and Po wipe the ground with Temutai and his men. Yao's kung fu moves, particularly the multiple enemy hypnotic chi-blast, are also incredible, particularly for TV animation.
In "Monkey in the Middle", pretty much the whole sequence in the mill:
Po taking down Ke-pa's demon army with the Hero's Chi and sealing them back in the underworld. He follows this up by turning Ke-pa's attempt to kill him around and blasting the Hero's Chi straight down his throat to destroy him.