Because of Abilities, Movesets, and Items being nonexistent/changed in LGPE, this is what I think. :P
First, lets compare the Mega Forms:
Mega Charizard X:
- It is the Fastest Dragon type, and is probably better than Dragonite.
- The best of 3 Pokemon that receive STAB on physical Fire type Moves.
- Can be run with Both Physical and Special Moves
- Better Defensive Typing
Mega Charizard Y:
- Has one of the Highest Base Special Attacks.
- Has less Defense, more special Defense.
- Receives Flying Stab.
I recommend Mega Charizard X because you can run it split so that it can take out more threats, it is the fastest Dragon type, and has a better typing allowing it to survive more hits.
Therefore, a Naive or Hasty nature is probably great for Charizard. You can Use Naive/Hasty/Modest/Timid for both Charizard's Megas, because they can both Utilize Special Attack and Speed, but Adamant and Jolly can only really be utilized by X alone.
I recommend a Naive Nature because It allows you to utilize Mega Charizard X's physical Bulk, as well as its Attack, Special Attack, and Speed stats. And if you want to use Mega Chariard Y instead, you can switch very smoothly. Your Special Defense won't be as good, but that doesn't matter much as Charizard Y is better offensively in-game and in Let's Go's competitive scene.
Hope I Helped!