My favorite Mega competitively, huh? .....HEHEHHEHEHEHHEHEHEHHEHEHE
My favorite Mega is easily Mega-Blastoise. Mega Blastoise has an amazing ability, Mega Launcher, which boosts those Aura and Pulse moves by 50%. Mega Blastoise already has a very high maximum Special Attack, so with a Pulse and Aura set with max Special Attack, the power of its Launcher moves will be amazingly high... Also, Mega Blastoise is pretty bulky on the defensive sides too. So this Mega is both offensive and defensive on one set. Mega Blastoise is slow, I admit, but it is tanky enough to take those super-effective hits, and strong enough to strike back. Also, Mega Blastoise has a great specially offensive movepool, too, and it can have the Pulse and Aura moves. It can even work as a Rapid Spinner, too, and since it's bulky enough to take this, it will at least get a chance to spin the Rocks away. Also, the Pusle and Aura mvoes are all different types, so if he uses them in one set, he can offer great coverage. Anyways, I just love Mega Blastoise overall. It's an amazing Pokémon.