There is really no really is no right or wrong nature for charizard just think of one that is good for yours. Like my charizard has a hasty nature, but I think if you want a good nature pick Brave or relaxed(those might be good or not).
If you want to:+sp atk but -atk is MODEST. +sp atk -def is MILD, ect. if you to see the appropriate nature of your pkmn, just tell me what you want to up!(when you up ne of ability, you have to down other,so,if you don't want to down one just for up other,take:HARDY,DOCILE,SERIOUS,BASHFUL,or QUIRKY)