Something I've been playing around with
Evs: 252 HP, 176 SpD, 80 Def
Despite its typing giving it some common OU weaknesses (Ice, Dragon and Fighting to name some) Hydreigon has got some underrated 92/90/90 defenses, when coupled with its ability Levitate and resistances to Water, Grass, Fire, Electric, Dark and Ghost and an immunity to Psychic, Ground and both Spikes and Toxic Spikes, it can make a fairly good and useful defensive Pokemon.
Dragon Tail: A great phazing move, works especially well with entry hazards and gets STAB.
Substitute: Works well with entry hazards and Dragon Tail, protects Hydreigon from status such as poison.
Roost: Helps keep Hydreigon healthy, thus giving it more opportunities to wreak havoc from behind a sub.
Flamethrower/Earth Power/Dark Pulse/Toxic/Thunder Wave: Flamethrower keeps Bug, Steel, Grass and Ice types at bay. Earth Power is mainly for Heatran and Tyranitar although Dragon Tail can just phaze them both out. Dark Pulse is another STAB option for those that are worried by Psychic and Ghost types such as Gengar, Alakazam and Trick Room Reuniclus. Toxic is for common walls like Jellicent, Vaporeon and others. Although Thunder Wave might seem like an odd option it can halt sweepers who rely on their speed such as the Musketeer trio, Lucario, Infernape, Choice Scarf Scizor and so on.