3x21 - Avatar: The Last Airbender

Aang defeats the Fire Lord by bending the elements within the Fire Lord and removing his firebending abilities. After Zuko is taken down protecting Katara from Azula, Katara joins the fight and manages to restrain Azula. Afterwards, Zuko is crowned Fire Lord and promises a better future for the world, while Aang takes his place as the next Avatar. Katara finally shows her love for Aang as they kiss under the setting sun.