Modest Charizard is always good, or, if you want to capitalize on its best stat, Speed, go Timid.
Sunny Day works wonders with Charizard, but it may prove more beneficial to have something else set it up for him.
Here's my favorite Charizard moveset.
Flamethrower
Sunny Day
Solarbeam
Dragon Pulse
Dragon Pulse works well with the STAB move and wall killer, providing Charizard with some coverage against a very commonly used type that he'd otherwise not be able to hit for much damage at all. Substitute can be used, but 'Zard is too frail to use it well, and it would buy him one turn at most. Even if you do set it up, you won't be any better off unless you have a move that isn't weak against your enemy.
If you're using something else to set up Sunny Day for him, you can stick Substitute in that spot, or EQ/Shadow Claw if you want to go the mixed sweeper route.
I recommend maxing out his Special Attack and Speed, and putting the other six EV points into whatever defense stat.