Charizard = Choicizard
I think Charizard has a lot of choices. There is no overwhelming obvious choices, but a lot of plausible choices.
Reminders:
Rock = 4X damage
Water/Electric = 2X damage
I will start with abilities
...
Both of these could be used like on multiscale dragonite
Filter = reduces supereffective damage by 1/4
Multiscale = 1/2 damage when at full HP
These would not be as useful, but good for surprise
Drought = Boosts fire 50% and negates waters effectiveness, but also increases Thunder accuracy. Easily out weathered by slower inducers.
Water Absorb = heal 1/4 HP when hit with water
Dry Skin = heal 1/4 hp by water, fire 25% more damage
Storm Drain=Boost Special Aattack when hit by water
Lightning rod = Electric boosts sepcial attack 1 stage.
Magic bounce= Reflects any moves that change status effects (bar Memento), status conditions, entry hazards, Taunt, Encore, Disable, and all other pseudo-conditions, and provides an immunity to said effects
Serene Grace = good choice if you want to capitalize on speed for flinch
I am sure there are others, but these are the oens that came to mind.
Next moves
If we are limiting the EVs to 510, most the smogon guides say go with 252 SA and 252 Speed. I would go with that, but I would not mind testing out a 252 speed 252 HP, a 252 SA 252 HP, and a 252 HP 132 SA, 120 speed charizards and see how they perform vs the 252 SA 252 speed. Especially if the ability helps it to be longer lasting and you have tailglow or quiver dance. Especially if you go with Spore.
I would go Timid if you go with tail glow or Modest with Quiver dance.
Without testing them, I would say I would rate them:
1)Multiscale with spore, Quiver dance, and modest
2)Serene grace with Airslash, timid, and maybe throw in roost vs a boosting move
3)Dry Skin for surprise factor, tail glow, timid.
I forgot about item, but probably:
1) Leftovers
2) Sitrus berries
3) Expert belt?
4) Flying gem
5) Life orb
I think fire gem might be overkill
Sorry did not put as much thought into item.