I prefer Draco Meteor/Surf/Dark Pulse/Fire Blast on the specs set. Dark Pulse is fairly underrated and is usually passed over for earth power, but it utterly destroys reuniclus; even if you switch into a CM it does over 80%. And people are still using Jellicent for some reason too. Earth Power is only really good for heatran, who could easily be holding a balloon and you have surf for him anyway.
I also prefer modest because Hydreigon's fantastic typing means that alot of stuff will be switching out of it, which basically negates the need for speed since you'll be hitting the switch-in. Even with modest, it still outspeeds a bunch of stuff, and alot of key stuff that it would outspeed with timid is commonly scarfed.
You'll be using Fire Blast a lot because people switch Ferrothorn in like it's their job.
Definitely one of my favorite gen 5 pokes and my favorite dragon so far. I just love its typing and insane coverage.