So as lots of people know, Charizard is a fan-favorite. But why? Is it nostalgia? When I asked my older friends they said it was the design. And then I thought, "But Charizard's design isn't even that great." I mean, it's got a good moveset, but it's design is literally just an orange dragon with a flame on its tail. So, what's up with Charizard?
As you know, Charizard is a starter from gen 1 so for long-term fans, it's likely of the original starters they chose from. And for many Charmander (Charizard's first-stage evolution) was their choice. I can't say for certain why each person that chose this starter, but it could be for one of multiple reasons such as they preferred it's design over squirtle and bulbasaur or it's name sounded cooler to them. After this it mainly became a favourite due to nostalgia but other contributing factors may include that people found use for it on their journey and remember it as a good Pokemon in gen 1.
So basically it is mostly it's a mixture of nostalgia from choosing it as their first Pokemon (or first for that region) and other reasons differing from person to person. Hope I helped!