For me, I always use my mega Kangaskhan. Parental bond is just amazing especially if he has a good attack. Move wise; power-up punch raises his attack by two stages rather than one. Big hitter = 2x as fast
However, I also love using Mega Altaria. Changing from a dragon/flying to a dragon/fairy makes it a formidable opponent. Move wise if it has cotton guard you'll have a tough time removing it from play.