This fully depends on which form and which set of Charizard you are using, so I will provide natures and sample movesets for every viable nature.
Standard Special Attacker: Timid (+Spe, -Atk)
Charizard has base 100 Speed, meaning it hits the sweet spot that outspeeds many common Pokemon, such as Great Tusk, Landorus-T, and Tapu Lele. To make the most out of that Speed stat, a Timid nature is used, which lowers Charizard's useless offensive stat in Attack in return for maximizing Speed.
This nature can be used with base Charizard, along with Mega Charizard Y, who is just a stronger special attacker and fills the same role.
Charizard @ Heavy-Duty Boots / Charizardite Y
Ability: Blaze
Tera Type: Fire
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Flamethrower
- Focus Blast
- Scorching Sands
- Roost
Physical Attacker/Sweeper: Jolly (+Spe, -SpA)/Adamant (+Atk, -SpA)
This is for the same reasons as before, just that since Attack is now the preferred offensive stat instead of Special Attack, Jolly reduces Special Attack to accomodate for it. However, if Dragon Dance is used in the set, Adamant can be used to maximize damage output. Because Dragon Dance already boosts Speed, Charizard can afford to sacrifice a bit of its Speed and use Adamant, which boosts Attack instead of Speed.
This nature can be used for physical attacking Charizard running something like Dragon Dance or Belly Drum, as well as Mega Charizard X, a powerful physical Dragon Dance sweeper. Note that normal Charizard rarely runs physical sets, but certain teams can find a place for it, especially in lower tiers.
Charizard @ Heavy-Duty Boots / Charizardite X
Ability: Blaze
Tera Type: Fire
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly/Adamant Nature
- Swords Dance/Dragon Dance
- Flare Blitz
- Dual Wingbeat
- Earthquake
Bulky Burn Spreader: Calm (+SpD, -Atk)
With 78/78/85 bulk, Charizard is actually respectfully bulky, and it can use that bulk to spread burns with Will-O-Wisp and heal off damage with Roost. Because burn halves the power of a Pokemon's physical attacks, Charizard should maximize its Special Defense to deal with special attacks better.
Charizard @ Heavy-Duty Boots
Ability: Blaze
Tera Type: Fire
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
IVs: 0 Atk
- Flamethrower
- Will-O-Wisp
- Defog
- Roost
The movesets above are written with regards to Generation 9 metagames and Generation 9 National Dex, although these natures can definitely be applied to earlier generations. For example, special attacking Charizard in Generation 5 does not have access to Scorching Sands but does have access to Hidden Power Grass, along with viably running Air Slash. In that case, the nature of Timid is still the best nature, it's just that the moves are different.
Hope I helped!