I've used a ton on showdown, But in game I've only used Blastoise, Tyranitar, Lucario (duh) and Banette. But Pinsir is the king of my showdown megas, Followed closely by Mawile.

Mega Charizard Y- 6-
Mega Aerodactyl- 7
Mega Scizor- 13
Mega Gengar- 21
Mega Alakazam- 9
Mega Banette- 17+
Mega Venusaur- 17
Mega Aggron- 10