Well charizard runs a wild diversity of sets, however most of them tend to be special attackers. So I would recommend running the Timid Nature on Charizard as it will enable you to be fast as well as have a great offensive presence.
Special attacker and recommended Natures
- Timid for the standard sets
- Modest/Timid for scarf sets
However it must also be noted that physcial sets are very good too. For this reason I would say Adamant or Jolly Nature if you are planning on using moves such as Flare Blitz, Earthquake etc.
Physical attacker Natures
- Jolly Nature for standard sets
- Adamant/Jolly Nature for choice physical scarf