I would say Flygon. You can switch it with Haxorus for competitive battling though, but in-game it is what you are looking for in a dragon type Pokemon. I used it myself in black2 and it served me well. It outspeeds almost anything if not anything in-game, including Haxorus and Hydreigon. It has a decent attack stat and acces to fire type moves, something that Haxorus lacks. Its powerfull STAB earthquake is always reliable and you can always teach it dragon claw and later Outrage through the move tutor. Its biggest flaw is its 4X weakness to ice type attacks, but it can usually handle them good if the ice type is not holding ice shard. I would say go for Flygon.
Hope this helped you with your decision a little :)