The mighty Hydreigon is a fearsome Dark/Dragon-type with stats putting it in the range of "pseudo-legendary", meaning its combined stats are equal to Mew, Celebi, etc. and are stronger than, say, the trio of legendaries in Black 2 and White 2. In order to unleash that power, though, you need to level up a Zweilous (pronounced ZV'AI-liss, and is a play on the German word zwei, meaning "two" — in fact, ein and drei are both German for "one" and "three" respectively) all the way up to level 64 before it will evolve. Not only that, but it needs over 327,000 EXP to reach that level, since it levels up so slow; by comparison, your starter Pokemon needs only 259,000 or so EXP to reach the same level. Since the final battles in the game use only high level 50's, you're probably going to be stuck with a Zweilous at that point unless you do a lot of extra training.
Zweilous' stats leave much to be desired, but for a middle stage, they're not too bad. It has ok Attack, average defensive stats, and some lackluster Speed. As a Zweilous, it has the Hustle ability, which raises the power of its Physical-based attacks by 50%, but decreases their accuracy down to 80% of its base accuracy. Once it finally evolves into Hydreigon, it has amazing Special Attack (not quite Chandelure or Espeon level, but still very high), great Attack, solid defensive stats and HP, plus really great Speed as well. Its stats truly are comparable to legendary Pokemon's stats. Not only that, but it ditches its Hustle ability for the much more practical Levitate, granting it a free immunity to Ground-type attacks. This grants it two immunities (Ground and Psychic), on top of six additional resistances (Fire, Water, Grass, Electric, Bug, and Ghost), which means it is actually comparable to many Steel-types as far as how many resistances it has. Despite being a Dragon-type, it does not have the typically characteristic quad weakness to Ice-type attacks, so it may be able to handle an Ice-type attack and live to tell the tale.
When you catch your Zweilous, it'll have both Dragon Pulse and Dragon Rush to work with. Both are great Dragon-type attacks; Dragon Rush is powerful thanks to Hustle and Zweilous' superior Attack, but the accuracy of just 75% before factoring in Hustle means it will miss very often, so you may actually be better off with Dragon Pulse. Body Slam is another attack in its arsenal from the start, which hits decent for a Normal-type attack, plus it has a chance of paralyzing the foe. It gets Hyper Voice right before evolving at level 64, which is a strong 90 power Special-based Normal-type attack. Lastly, it gets Outrage at level 79 as a Hydreigon, which is strong, but Dragon Pulse is usually better, despite being a little weaker (its Special Attack is much higher than its Attack, so it's not too much weaker). Either form can be retaught Crunch by the Reminder Girl for a Heart Scale, although it's really only Zweilous who benefits from it (it's definitely recommended, as it has a base 100% accuracy). Hydreigon can relearn Tri Attack, which has 10 less power than Hyper Voice, but has a 20% chance of inflicting either Burn, Freeze, or Paralyze to the foe. It isn't really necessary, though, since Hydreigon has so many attacks to consider.
Now, regarding TMs/HMs, I'm going to go on the assumption that you won't have Hydreigon until post-game. I'll start with just Zweilous. Sadly, Zweilous doesn't have access to a lot. Dragon Tail is literally the most exciting TM move it can learn. Work Up can raise both of its stats, but it actually knows that when you first catch it. Zen Headbutt, Aqua Tail, Dark Pulse, and Earth Power are options to consider and cost either 8 or 10 Blue Shards, but none are really must-haves for it.
Ok, now the fun part: Hydreigon's TMs/HMs. Hydreigon has better Special Attack than Attack, but its Attack is high enough that it can use either Physical- or Special-based attacks just fine. Flamethrower, Fire Blast, and Surf are all very solid options on the Special-based attacking side. Flash Cannon and Focus Blast can also be used after beating the game, too (I'll mention ALL post-game TMs in this, too), though it isn't really that amazing. Earthquake, Acrobatics, Stone Edge and even possibly U-Turn make for decent Physical-based attacking options depending on your strategy and typing needs. Acrobatics in particular tends to be nice to combat Bug- or Fighting-type Pokemon looking to exploit its weakness. Fly can be used instead of Acrobatics if you'd rather use a held item, plus Rock Slide can be used instead of Stone Edge if you'd rather have more accuracy at the cost of power. Some support moves include Reflect to halve the Physical-based damage received by your team for a couple of turns, along with the previously mentioned Work Up to increase the potency for both of its attacking stats. It also gets a TON of Move Tutor moves! Signal Beam for 4 Red Shards is a cheap, underwhelming option, but it does provide some Special-based Bug-type coverage at least. Dark Pulse for 10 Blue Shards is really a must-have for Hydreigon, especially when combined with Dragon Pulse. Earth Power for 8 Blue Shards hits even harder than Earthquake and, for better or worse, hits only one Pokemon. Heat Wave is stronger than Flamethrower, weaker than Fire Blast, but hits all adjacent foes (foes only, woohoo!) in Double or Triple Battles, so it may be a better option for you, too, if you're willing to part with 10 Yellow Shards. While it isn't exactly the most inviting support Pokemon in the world, Tailwind can be taught for 10 Yellow Shards as well, which will raise your team's Speed by 50% for the next couple of turns. Additionally, it can learn Draco Meteor by returning to Drayden in Opelucid City after beating the game and having you teach it to your Hydreigon. It does a meteoric amount of damage, but sharply lowers Hydreigon's Special Attack afterwards, meaning you'll want to switch out afterwards or just rely on Physical-based attacks.
The attacks you'll definitely want to have on your Hydreigon are Dragon Pulse and Dark Pulse. Beyond that, just switch things around depending on your team or what types you need coverage for. Dragon Pulse + Dark Pulse + Flamethrower/Heat Wave + Earth Power gives you a super effective attack against a ton of Pokemon and ensures a powerful Dragon-type attack against anything you can't hit with a super effective hit. But you can use pretty much whatever you want with it. It's about as versatile as it gets.
Really, you can't say Hydreigon is anything short of amazing. It's the strongest non-legendary Dragon-type in the Unova Dex. It has great stats, is sturdy as heck, can learn a TON of moves and actually use them, plus it's got a ton of resistances. The real downside to it is that it takes forever to actually get to the point where you are free from the infinitely weaker Zweilous. There's still tons to do after beating the game, and since you have only about 14 or so levels to go before evolving unlike the 22+ in the original Black and White, it is a much more credible option now than it was back in the first pair of games. If you have the patience to invest into training one, I definitely recommend it. What I'd recommend even more, though, is waiting until after you beat the game before you actually start training it, because Zweilous will not be very helpful then whereas the extra levels not wasted on it will help out the rest of your team. Plus you have access to more training spots after beating the game to speed the process up substantially. Because of that, I have to knock its rating down to 4½, but it'd easily be a solid 5 if it just evolved at, say, level 55.