Hydreigon has a terrific special movepool and base 125 Special Attack to back up its good offensive typing. Defensively, it isn't too much of a slouch either, with 92 / 90 / 90 defenses, allowing it to take powerful attacks without Focus Sash. Unfortunately, its Speed leaves a lot to be desired, as it leaves Hydreigon outsped by a large portion of the metagame.
Dragon Pulse gives Hydreigon a solid STAB attack that only a few Pokemon resist, all of which are hit for super effective damage by Hydreigon's Fire-type move of choice. If you aren't concerned about Hydreigon's capability to maintain a continued long-term offensive, Draco Meteor is also an option for a much more powerful initial hit. Dark Pulse is there for a secondary STAB attack. While it provides no extra neutral coverage, it does allow Hydreigon to hit Psychic- and Ghost-types such as Reuniclus, Beheeyem, Cofagrigus, and Musharna much harder. Dark Pulse also comes with a 20% chance to flinch the opponent, which can be game changing in some situations like potentially stopping Trick Room set up. The choice between Flamethrower or Fire Blast is the classic dilemma of accuracy versus power, and depends on your personal preference. It's worth noting, however, that Fire Blast assures a 2HKO on Amoonguss even when using a Timid nature, while Flamethrower never 2HKOes if it invests enough EVs in Special Defense and HP. Whichever Fire-type attack Hydreigon has, it will help it with the few Steel-types present in 2011 play, and scores super effective hits on the numerous Grass- and Bug-type Pokemon.
The choice of the final move really comes down to what type of team Hydreigon is playing in. If you really want to try and stop set up, Hydreigon can run Taunt and attempt to keep Trick Room, Tailwind, or rain from being set up, but it won't have any effect on the Prankster Tailwinds coming from Whimsicott and Tornadus. Remember, however, that Mental Herb now cures Taunt, and so Taunt is not a completely reliable way to stop Pokemon from setting up. If you are pairing Hydreigon with a Water Absorb user such as Jellicent, Surf can be a very valuable attack for extra coverage against sand teams. It is also useful as a spread attack that can finish off a weakened enemy while still dealing damage to its partner. It also allows Hydreigon to heal your Jellicent, possibly powering up its Water Spout. Finally, if your team doesn't fit either description above, Protect can be run as as consistently useful move.Team Options & Additional Comments >>>
In terms of other notable coverage attacks, Hydreigon doesn't really have anything else of note, but it does have several other support options at its disposal, namely Thunder Wave, Dragon Tail, and Reflect. Dragon Tail is mainly useful for stopping Trick Room. Hydreigon has a competent base Attack stat that could be used to run a mixed set, but there really aren't enough Pokemon with lopsided defenses to warrant the overall drop in power. One advantage of a mixed set, however, is the ability to run Draco Meteor without worrying about the Special Attack drop since Hydreigon will be able to fall back on its physical attacks.
As far as items are concerned, there are a lot of noteworthy alternatives. Hydreigon can use Dragon Jewel to have a one time boost to its Dragon-type move of choice, but if Hydreigon attacks even twice, the boost from Life Orb will have been more beneficial. Life Orb's ability to boost the power of all of Hydreigon's formidable attacks is also important to consider. If you really want to assure Hydreigon can take a hit, Focus Sash is certainly an option, but usually, Hydreigon's bulk will be sufficient. Finally, if you want to run Draco Meteor without worrying about the Special Attack drop, Hydreigon can run White Herb to negate Draco Meteor's effect the first time.
Checks and Counters
Defensively, very few Pokemon can truly counter Hydreigon because of its great coverage and sheer power, especially since almost all of the most bulky Pokemon are weak to Dark-type attacks. However, Hydreigon's middling Speed and Dark typing leaves it vulnerable to being defeated outright by many faster enemies. Volcarona outspeeds Hydreigon and can OHKO with Bug Buzz. Among Fighting-types, Terrakion, Virizion, Cobalion, and Mienshao can outspeed Hydreigon and OHKO it. Conkeldurr can deal serious damage to Hydreigon with Mach Punch, or OHKO it with Hammer Arm, without incurring too much damage to itself. The last Fighting-type worth mentioning is Scrafty, which can easily 2HKO with Drain Punch, while not being 2HKOed itself with the healing from Drain Punch factored in.