Ash uses Charizard in the Orange Islands championship battle in the Pokémon TV series. Charizard defeats Drake's powerful Dragonite, helping Ash win the Orange League championship.
The Orange League is different from the standard Gym-and-League structure used in Kanto. Ash challenges four Gym Leaders across the Orange Islands and then faces the Orange Crew Supreme Gym Leader, Drake. Charizard's victory over Dragonite is the decisive final battle of that competition.
Charizard had previously been difficult for Ash to control, making the championship result especially significant in the character's development. Its victory also established Charizard as one of Ash's strongest and most recognizable Pokémon. The Orange Islands storyline takes place after the original Kanto League and before Ash's journey through Johto.