Oh how the mighty have fallen. What was once a premier threat back in gen 5 thanks to excellent stats, none of which were less than 90, superb typing both offensively and defensively, and an incredible attack movepool that gave Hydreigon everything it could need and more, Hydreigon has since taken a tumble.
So what happened? This happened:
The advent of the new Fairy type which was strong against Dragon AND Dark types suddenly meant Hydreigon joined the 4x weakness psuedo legendary club alongside Dragonite, Tyranitar, Salamence, and Garchomp (Also Kommo-O two generations later). And with so many competitive teams now running the spammable Fairy types, Hydreigon suddenly found its usage dwindling even in spite of gaining a few new tools like Dragon Dance and Nasty Plot in subsequent generations. That is not to say Hydreigon is now useless. It is far from that no thanks to the previously mentions pros. Just be weary of that major Fairy type con and Hydreigon will serve you well.
Item: Life Orb
Nature: Timid (+Speed -Attack)
EVs: (HP 4) (Sp.Attack 252) (Speed 252)
Dragon Pulse/Draco Meteor
Flash Cannon/Fire Blast/Earth Power/Hydro Pump
A Nasty Plot sweeper set. Set up Nasty Plot, spam Dark Pulse as your main STAB, Dragon Pulse is for anything that resists Dark Pulse though you can sub in Draco Meteor if you want the extra power and can calculate around the stat drop, and finally we have your choice of coverage. Flash Cannon counters your glaring weakness to Fairy types, Fire Blast punishes the likes of Ferrothorn and the Bug/Steels, Earth Power has excellent type coverage, and Hydro Pump sound similar to "Hydra Pump" for lols and specific coverage. Life Orb grants extra power, and the EVs go into special attack but mostly speed given the weird speed tier Hydreigon sits at.
Item: Life Orb
Nature: Jolly (+Speed -Sp.Attack)
EVs: (HP 4) (Attack 252) (Speed 252)
Scale Shot/Outrage/Dragon Rush
Iron Tail/Earthquake/Fire Fang/Zen Headbutt
While the weaker of its two attack stats, 105 physical attack is still good. And with newfound access to Dragon Dance, Hydreigon can now patch up its weird speed and sweep like a Greek mythological monster. Set up Dragon Dance, use those hydra fangs well with Crunch as your main offense (Throat Chop is also doable if you fear sound moves) and slap on a Dragon STAB of choice. Scale Shot has inconsistent power but offers an extra speed boost (Possibly even letting you run an Adamant nature), Outrage has the most power but locking yourself into a move that confuses you after and can be sponged by Fairy types is a major gambit, and Dragon Rush is the moderate option with more consistent damage and power but at the cost of accuracy. Finally we have your choice of coverage with Iron Tail being your answer to Fairy types (Just watch that accuracy again), Earthquake has the best overall coverage against many types, Fire Fang is a giant "Bite Me" to Ferrothorn and the Bug/Steels, and Zen Headbutt is there for the Fighting types who think they will only have to worry about your Dragon STAB. Once again, offense and speed investment to the max with Life Orb providing the additional damage boost.
So don't let the intro of Fairy types be a deterrent to using Hydreigon. This ol' girl can still strike fear into the hearts of competitive players.