Verified answer

1. Hydreigon's answer is wrong. Mega Lucario is banned by Smogon players, but it is still allowed in the Battle Spot and outside of Smogon.

2. Mega Lucario is pretty tough and has good speed, but there are many scenarios in which it will fail to deliver. Case in point, I've never won a battle using it.

3. Mega Gyarados has the Mold Breaker ability but doesn't really get any powerful boosts. It's better to give him the Moxie ability and have him serve a purely offensive role, saving your mega evolution for someone else.

4. Mega Charizard Y with Timid or Modest nature and good IVs/EVs is really fast and powerful, but is also very frail. If you want him to work, send him out with a defensive Pokémon that can use reflect and light screen, like a Prankster Klefki, or another, faster sweeper type. Charizard and Heracross work well together. In singles, he's not really useful.

Right now my favorite is Mega Aggron, who has some great defenses and can use Thunder Wave to paralyze the opponent so he gets to move first.

Mega Luario is banned, sadly.

I would invest in Charizard X as he is bulky and can be used as a special or physical attacker.

MegaGyarados can't take a playrough very well unless you invest evs into his defense.

I have personally been using Mega Scizor, he can eat up three fire fangs which allows me to put up three swords dances then I completely destroy a team with bullet punch as his defense and attack are amazing.