In short, I'd go for: Flamethrower / Focus Blast / Dragon Pulse / Substitute
There are 3 possible fire moves you could use for a special Charizard:
- Flamethrower (power 95, accuracy 100)
- Heat Wave (power 100, accuracy 90)
- Fire Blast (power 120, accuracy 85)
It depends on whether you want power or reliability (personally I go for reliability). Or, if you're expecting to switch in, KO, then switch out, Overheat is a great option. For the other moveslots:
Focus Blast takes care of Rock pokemon, among others.
Dragon Pulse is very powerful and takes care of the unpredictable nature of dragons - they could have Thunderbolt, Aqua Tail, Surf, Stone Edge or other super-effective moves.
Ominous Wind is a good one, to cover Ghost/Psychic. Substitute is always an option. Air Slash could be used for a STAB boost, but it doesn't cover anything extra than you already have.