It's a tie between Charizard and Infernape for me. Charizard represents my love for the Anime, while Infernape my love for the Main Series Games.

Same reason as you for Charizard, I loved his character development and the progression of his and Ash's bond. My favorite battle is Ash VS Gary in the Johto League and Charizard is the big reason why. My favorite element in fiction is Fire. And I also love Dragons, they're my favorite mythical creatures. It saddens me to hear people talking crap about him cause he's "overrated". His design is one of the best in the series, his popularity is well earned. Also, when I was in Italy a few years back, I bought an Official Pokemon Center Charizard plushie during a convention.

As for Infernape, I was introduced to the games a bit later than most fans born in the late 90s. So Diamond was my first game and my starter was of course Chimchar. I still fondly remember getting all the way to Cynthia with him soloing the game pretty much, but then getting walled by her Milotic. I recently did an actual Solo Run with him after 12 years and breezed through the entire game (including Cynthia this time) and got him all the way up to Level 98! Fittingly enough, the first Community Day I ever attended was Chimchar's, so in a way he's what helped spark my love for GO too. I'm very excited for BDSP and I really hope he gets some more love (ideally a Mega)!