I've seen a lot of 'best' questions answered with BST. Well, it's time to change that.
There are too many powerful dragons to list, so I'll use the main ones. Hydreigon, Dragonite, Garchomp, Salamence.
Notable Moves: Draco Meteor, Dragon Pulse, Flamethrower, Outrage, Crunch, etc.
Pretty powerful stats, decent Speed, and large attacking stats. Pretty much a standard sweeper.
What sets Hydreigon apart from the rest (IMO) is his different typing. The 2x, rather than 4x weakness to Ice, a weakness to Fighting, an immunity to Ground. This is what helps Hydreigon make a name for itself.
And here's Smogon's analysis:
There is no Pokemon more violent than Hydreigon; it's well-known for attacking anything that so much as moves. This is a fairly apt observation, since in competitive play, Hydreigon will do everything in its power to destroy its target. Hydreigon has a scary combination of high powered STAB moves, high offensive stats, and a wide array of coverage moves that target everything in OU for at least neutral damage. While other Dragon-types come close to being uncounterable, Hydreigon drops all pretenses and actually IS uncounterable. To put it bluntly, Hydreigon is flat out impossible to wall, and if you think that it can be beaten by a Steel-type, think again. It can obliterate every single Steel-type in OU given the chance, and turn them into a fine powder with its large selection of coverage moves. If that wasn't enough, Hydreigon has some pretty decent defensive stats for an offensively oriented Pokemon. 92 / 90 / 90 is nothing to laugh at, and it will usually take a strong super effective move to take it down. Access to Levitate and Roost also means it's hard to wear it down with hazards. Fortunately (or unfortunately?), Hydreigon has a fairly crippling shortcoming that prevents it from utterly destroying teams: its Speed. Due to its mediocre Speed, Hydreigon is outsped by the majority of offensive Pokemon in the tier, meaning it will often be forced out after a KO. This is the fundamental flaw that keeps Hydreigon from being a staple on most teams. It has awesome power, but being forced out after a KO is a clear detriment. Being weak to common priority moves such as Mach Punch and Ice Shard doesn't help either. However, none of that really matters if you're committed to making Hydreigon work. When it comes to pure wallbreaking, none can match Hydreigon's ability to eliminate everything in its path.
Overall, a powerful coverage sweeper that is set apart from the pack by its coverage moves and typing.
Notable Moves: Draco Meteor, Hurricane Outrage, ExtremeSpeed, Fire/Thunder/Ice Punch, etc.
What really defines Dragonite is its ability, Multiscale. Multiscale halves the damage taken by moves when Dragonite is at full HP, a real boon that allows him to grab a few extra boosts or survive an otherwise fatal attack. Aside from that, Dragonite has deadly Attack, decent Sp. Attack, and good Defenses, which combined with Multiscale are enough. However, base 80 speed will always be Dragonite's downfall. Even when he is Scarfed, faster Scarfers can outspeed and kill him.
In BW, Dragonite rose from the shadows of its competitors due to being gifted with the rare Multiscale. With Multiscale, Dragonite has gone from an outclassed niche Pokemon to one of the most dominant offensive and defensive threats in the metagame. It can fill almost any role and is one of the few Pokemon that can claim to work on any team due to its endless movepool and well-distributed stats. However, residual damage, particularly a nasty Stealth Rock weakness, limits Multiscale's effectiveness. A slightly below-average Speed holds it back as well, though Multiscale helps mitigate this flaw somewhat. Speedy threats such as Choice Scarf Genesect and Thundurus-T don't do it any favors either. Regardless of these downfalls, Dragonite defines the OU metagame with its great bulk and power, and will continue to do so for the foreseeable future.
Overall, a bulky powerful sweeper that can do a ton of damage if given the chance.
Notable Moves: Outrage, Earthquake, Stone Edge, Dragon Rush, Crunch, Swords Dance
Garchomp is the speedy dragon. With base 102 Speed combined with 130 Attck, he can easily rip holes in any team. Swords Dance furthers this, as it will boost Garchomp's power to 6-0ing levels. However, Ice Shard can take down Garchomp easily, as can any powerful physical wall. Nevertheless, Garchomp is still incredibly powerful.
The generation shift brought Garchomp no notable moves and basically left it the same old Garchomp. This, however, is not a bad thing by any means, since Garchomp is just as as threatening as ever with its great base 102 Speed, which allows it to outspeed a good portion of the tier, and awesome base 130 Attack. In addition, Garchomp also has excellent Dragon- and Ground-type STABs, which allow it to hit nearly everything in the game for at least neutral damage. However, Garchomp does have its shortcomings; its defenses, while still decent, pale in comparison to the Uber tier's powerhouses. Also, its great STAB moves can only carry it so far, and it will sometimes find itself walled quite easily. With the changes BW2 brought to the metagame, namely the reintroduction of Soul Dew, Latias and Latios have seen a major rise in their previously rock-bottom usage. This is a stumbling block for Garchomp, as they both easily outspeed and threaten it with their own powerful Dragon-type attacks if Garchomp isn't using a Choice Scarf.
Cool. Garchomp is the powerful, fast dragon.
Notable Moves: Dragon Dance, Dragon Claw, Outrage, Draco Meteor, Roost, Fire Blast
Salamence is all about power. With 135 base Attack, the highest yet and Moxie to boost this even further, Salamence is a powerful offensive threat in today's metagame. A large movepool and a more than usable base 110 Sp. Attack all contributes to Salamence's side. However a lack of bulk and a weakness to Stealth Rock brings Salamence down. However, Salamence is still a menace and a force to be reckoned with.
Salamence possesses an enormous offensive movepool, which, when combined with its fantastic offensive stats of base 135 Attack and 110 Special Attack, has the potential to tear through even the most prepared of teams. Despite not changing very much since DPP, Salamence still has all the tools needed to pose a major threat to the opponent, whether it be early-, mid-, or late-game. While Salamence does face competition from new threats such as Latios, Haxorus, and Hydreigon, it has its advantages over each of them.
Perhaps Salamence's greatest advantage over its competition is its versatility. Due to its ability to go physical, special, or even mixed between various sets, true counters to Salamence remain few and far between. Its defensive capabilities are not to be overlooked either. 95 / 80 / 80 defenses aren't too shabby, and in conjunction with reliable recovery in Roost and Intimidate to weaken physical attacks, Salamence is one of the most durable sweepers available. A purely defensive set is also more than viable due to these traits, which can utilize a new move in Dragon Tail in order to phaze enemies and rack up entry hazard damage. Overall, Salamence remains one of the most dangerous Pokemon in OU, and has the potential to change the course of any match in the blink of an eye.
Salamence is the power Dragon. 135 base Attack + Moxie + Outrage + potential Band = killing.
Hydreigon = coverage
Dragonite = bulk
Garchomp = speed
Salamence = power