Depends. If you need a physical attacker, go with Mega Charizard X. If you need a special attacker. go with Mega Charizard Y. Charizard-Y's ability drought gives it that solar beam usage, and with Charizard-X its ability tough claws allows for a lot of damage with physical attacks. So, go with which ever you need to perfect ypur team. Heres a moveset for both-
Hope I Helped!