Katara's not the only Waterbender you do not want to get angry. In the episode "Teers in the Hood," when Gi learns the identity of the gang member who shot and hospitalized her friend, she completely snaps and uses her ring to send him flying up a waterslide and down into a pool where she tries to drown him. Beware the Nice Ones, indeed!
Wheeler: Gi, you don't really want to do this!
Gi: Yes, I do!
Gi going all gangster when she takes the gun off Wheeler and intends to use it on the guy who shot King.
The end of the episode "Teers In The Hood." The images of civil rights leaders are one the most powerful and serious moments in a kids' cartoon even to this day.
The epic way Captain Planet defeats Captain Pollution the first time.
Captain Planet: (bearhugs Captain Pollution) EARTH! (drives them both through the Earth's crust) FIRE! (they reach the Earth's mantle) WIND! (flies back into the sky, swings Pollution around by his ankle) WATER! (throws him into the sea)
Captain Pollution: (Captain Planet retrieves him) Oh, have a heart, Captain Planet!
Captain Planet: Oh, did you enjoy your bath? Well let me hang you out to dry! (slams him down)
All the battles between the upgraded (and increasingly more ferocious) Boots and Buster in the episode "In Zarm's Way".
In "Tree of Life", Dr. Blight sprays herself with a magic sap that grants her superpowers even greater than Captain Planet's. While curb-stomping Captain Planet a second time, Wheeler suddenly tackles her. Of course, it has no effect and she easily throws him away. Blight notices her powers are waning and tries to refuel on more sap, but can't find it. Wheeler reveals he picked her pocket, then sprays Captain Planet with the sap. He gets supercharged and defeats Blight.