Assuming they both have maximum Speed, Garchomp will move first. As it have 333 Speed, which is max, it will outspeed the Charizard, even if it Mega Evolves. The next turn Charizard will move first, as it's Speed of 328 out speeds MGarchomps 311. The only way for Charizard to move first on the first turn, is if it should have the Choice Scarf equipped, as that boosts it Speed by 1.5.