After some thought, here it is...the ultimate set for hydreigon. There is no Pokemon that can afford to switch into this monster:
Hydreigon @ Life Orb
Nature: Naive/Hasty
4 ATK/252 SP. ATK./252 SPD.
Draco Meteor/Dragon Pulse
Superpower
Fire Blast/Flamethrower
Surf/Earth Power/Dark Pulse/Roost/Sub
EVs are self-explanatory: maximum speed and power. If you want superpower to be that much stronger you can shift some EVs into attack if you want.
There's a lot of options for this set and it all comes down to what you most want to destroy. Superpower is the only thing that should be carved in stone - it decimates blissey and tyranitar, who otherwise could wall the Hydra. Draco Meteor has much more obvious power, but Dragon Pulse can be used if you prefer the Hydra to stay in longer. Fire Blast is another obvious choice - roast scizor, forretress, ferrothorn, and other steel types who think they can wall you. You can use flamethrower if you're concerned about accuracy but you might miss out on important OHKO's.
The last moveslot is the toughest. With Dragon STAB, Superpower, and Fire Blast, you can hit anything in the metagame very hard, so the last moveslot is for additional coverage. For instance, Dark Pulse gets STAB and mauls stuff like Jellicent, Reuniclus, Mew, and Celebi. Surf hits hard and can function as pseudo-STAB in the rain. Earth Power can effectively end the threat of Magnezone or Heatran. Roost is there if you want to keep your hydra around longer, and sub can be used if you want to sweep and avoid status.
From the following calcs, it seems that Dark Pulse is a useful move for Hydreigon overall, but what do you think?