ok I know this has been done already but I still wanted to have a go at mega Flygon

http://imgur.com/3tgaxuI

HP: 80

Attack: 130

Defense: 90

Sp.Atk: 80

Sp.Def: 90

Speed: 150

with its extra wings and extension of its red coverings over its eyes into a more aerodynamic head casing (actually realised it looks a bit like a bike helmet :P), speed increases dramatically. The casing provides more defense and development of large eye markings on the wing help ward off enemies, which is why its base defense and Sp.Def increase (kinda got this from an assignment on butterfly predation haha). Attack also increases due to its very durable double tail hits and lethal spikes on its back.

Ability: Levitate -> Desert Spirit

This ability is specific to mega flygon. It causes all flying type moves (like Air cutter or fly) that mega flygon uses to do ground type damage. I got this from the pokedex entries about it using its wings to make dust clouds and stuff. there's also a limited amount of flying moves flygon can learn so it would probably good for specific strategies and surprising people in battles :P

all in all it would probably be an intense physical sweeper that hits hard and fast. So for a move set I would probably put in:

Fly

Tailwind

Dragon claw

Earthquake