I posted this in the creative/underrated sets thread a while back. It's my favorite set for Hydreigon this gen, as it catches a lot of players off guard:
Hydreigon @ Leftovers
Calm, 252 HP 4 SpA 252 SpD
Dark Pulse
Substitute
Toxic
Roost
Hydreigon possesses the typing and bulk to run an effective specially defensive set. Dark/Dragon grants him a bevy of resistances (Fire, Water, Grass, Electric, Dark, and the ubiquitous Ghost), and an immunity to Psychic moves. Ground type attacks will also be ineffectual against Hydreigon thanks to Levitate. 92/90/90 defenses are nothing to sneeze at, especially with full investment. For comparison, Zapdos's slightly inferior 90/90/90 defenses have allowed it to function as a formidable special wall since the Diamond and Pearl era.
Hydreigon's reputation as a threatening offensive presence tends to force numerous switches, which result in free substitutes. From there, Hydreigon can Toxic the opponent's switch-in and proceed to switch out, or stall with Roost and Substitute. With just two type resistences, Dark Pulse can often be spammed ad nauseum against slower foes, resulting in flinches and therefore wasted turns by an opponent. Although uninvested in this moveset, Hydreigon's exceptional 125 base Special attack stat will result in Dark Pulse inflicting considerable damage under most circumstances.
Steel types with high physical defenses pair well with this Hydreigon variant. They can threaten fairies, the bane of Hydreigon's existence, and absorb the Fire, Bug, Dragon, and Ice type attacks intended for him. In return, Hydreigon can switch in on Fire and Ground attacks aimed at his Steel type allies. A Ghost Pokemon can serve as a fitting 3rd member to this potential core, thanks to its immunity to the Fighting moves often thrown in Hydreigon's direction.