Ash's Charizard
Ash Ketchum has a Charizard that he originally obtained as a Charmander in Charmander the Stray Pokémon. The Charmander was abandoned and after a few hard nights of sheltering it, when the Charmander chose it's "true owner" it chose Ash. Charmander was immeadatly one of Ash's most used, and favorite Pokemon and became another member on the team. Its most notable move was when it battled a Primeape and used the extremely powerful Rage technique.
Soon after, it evolved into Charmeleon in March of the Exeggutor Squad and quickly lost its love and affection toward Ash. A few episodes down the line, it evolved again into Charizard in Attack of the Prehistoric Pokémon. From the time of its becoming Charmeleon to the Orange Islands episode Charizard Chills, Charizard refused to obey Ash. After Charizard Chills, however, it became one of Ash's strongest Pokémon, perhaps even stronger than Pikachu. This is due to the fact that Charizard was frozen by a powerful Poliwrath and Ash stayed up all night trying to help it.
In EP134 of the Johto League, Ash finally left his Charizard in the Charicific Valley with Lisa to train even harder and become more powerful than before. The Charizard can be called upon at anytime and has been used in a majority of major battles including the series' first double battle and more recently in AG136 First Battle! Battle Factory! where it took on the Frontier Brain of the Battle Factory, Noland's Articuno.