Mega Garchomp would have to be one of the greatest dragon-types available in Kalos at the moment, in my opinion. Its base Attack stat is one-seventy, which is a lot. Compared to its normal form, it also has better defensive stats. It may be a little slow, but that doesn't really matter. Mega Garchomp has access to a variety of moves as well, including Swords Dance and Hone Claws. After the usage of either of those moves, it should immediately be able to sweep your foe. (: Aside from this, Flygon is a pretty good dragon-type, too, and also Mega Charizard X.