I run my Hydreigon like this.
Hydreigon@Chopple Berry
Ability:Levitate
Modest Nature: +SpAtk, -Atk
Evs:252 Spe, 252 SpAtk, 4 def
-Draco Meteor
-Dark Pulse
-Earth Power
-Fire Blast
Chopple Berry is due to the fact that i end up dealing with pokemon who try to kill this thing with fighting more than Ice.
Fire Blast and Draco Meteor are self explaintory, Earth Power covers alot of elements, more than Surf, and is more reliable than Focus Blast which is too much of an "all or nothing" move to risk using, now as for Dark Pulse.
Maybe Dark's not a good attacking type, but Dark Pulse coming off a 383 SpAtk +STAB does some hefty damage to anything that doesn't resist the element, or has high special defense, not to mention it has a nice flinch factor which can be nice when it happens.
I really don't see the point of putting Superpower on Hydreigon unless it has some form of Atk investment (more than 4 points)
After all I once nailed a Blissey with a critical Draco Meteor when it switched in, then tore after it with a Superpower built Hydreigon, did Blissey die? No it did not, it held on, recovered and proceeded to kill me with Ice Beam.
If Superpower was really a godsend for Hydreigon, Blissey would've died, and while yes there weren't any entry hazards up, that critical Draco Meteor blew quite the chunk off of Blissey's health gauge so it wouldn't have mattered if they were involved.
Unless someone can find a way to invest more Ev's into Atk for Superpower then I don't think Hydreigon should be using it unless it's built as a pure physical attacker, or the team is heavily reliant on more than one entry hazard, because I don't see who else the phsyical attack can kill with builds that have most if not all of Hydreigon's Ev's in Special Attack and Speed.