Alright, lets start off with Garchomp
Pros
Very high Atk(at base stat of 170!)
Either Decent or Good in all of stats
Cons
Slower when Mega Evolved
Has a 4x weakness to ice type, meaning it will most likely NOT survive an ice type attack
Has a horrible abitiliy, Sand Force, which it needs to set up sandstorm to activate and wastes 1 turn
If you are planning to use Garchomp, use this example:
Garchomp
Adamant
Outrage-Gonna sweep with Swords Dance
Swords Dance-To set up
Earthquake-STAB
Dragon Rush-STAB, Replacement for Outrage
Ok, next is Charizard
Pros
Dragon fire type, great dual/mix up
Decent/Great base stats
Great ability
Cons
Has not that great base stats in Sp. Def and Hp, meaning it probaly wont survive a Earth Power
If youre choosing Charizard, heres an example:
Charizard X
Adamant
Flare Blitz- Main Move, STAB
Outrage- STAB
Dragon Dance-Sets up
Steel Wing- Beats up any opposing Rock type Pokemon
I would personally go for Charizard, as it is Faster than Garchomp and can learn Dragon Dance via Breeding and can sweep.