>>36493860The answer is the anime. Charizard is presented as a bamf in the anime. Ash's powerhouse. Not only is showcased as powerful, but it does way more than just fly around and occasionally breath fire. It does an exaggerated judo throw as its signature finisher, fights using its arms (which are slightly changed to be longer and better proportioned to its body). Then, you present these things to an impressionable young audience, and bam, you got yourself a fan favorite. For fuck sake, it's not rocket science that Charizard is a popular as it is. Give almost any other starter the attention it got at the height of pokemania, as well as dedicated specials, and they'd be just as popular.
