The perfect learnset for a charizard:
If it has solar power ability:
-air slash: STAB, may cause flinching, decent power, good accuracy.
-heat wave/flamethrower: STAB, good power, very good accuracy, powered up by solar power, flamethrower has better pp but heat wave can hit all opponents.
-solarbeam: very good power and accuracy, doesn't require 2 turns with solar power, great against rock types (due to the 4x weakness) and water types (2x weakness).
-counter/earthquake: counter would be great against a rock type causing an incredibly powerful counter-attack with double power, but only with a focus band/sash attached or rock moves could be lethal. And earthquake would be great agains electric types (2x weakness) and rock types too.
(earthquake might be a little better).
- if charizard knows counter, a focus band or focus sash should fit perfectly for a powerful combo.
- if not, leftovers can be good for restoring hp, charcoal to power (even more) up fire tipes, miracle seed to make solarbeam lethal, sharp beek to make air slash stronger, soft sand to increase earthquake's power, life orb, berries, etc.
Finally, iv's and ev's :
Personally, i'd rather train my charizard in special attack than in attack (knowing that solarbeam, heat wave, flamethrower and air slash are special), even though you should be aware of attack since charizard cannot learn a special ground-type move and earthquake is a fisical one. Also, instead of counter you can teach charizard superpower if you want to raise sp. atk. to it's maximum.
Speed is very important for a charizard, it must be pretty high. Train it.
The hp would probably require to be trained a little more, or instead raise defense more, if not rock-type attacks might, and will, be lethal.
Note: a charizard with solar power ability might be pretty rare, if don't have one, here could be a good learnset:
If you think electric types are uncovered, take the risk and, instead of one less turn of solarbeam, replace sunny day for earthquake.
The truth is there isn't a rule for this, everyone has it's own playing style and techniques. and every choice you take will be great if you train hard and wisely.
First post, sorry for the length :)
Hope this helps!