Garchomp all the way. With a killer typing, excellent movepool, overpowered Abilities and amazing stats,this is one of the most powerful Pokemon around.
It is the 3rd fastest Dragon, surpassed only by Latios and Latias, it can compete against Dragons of higher calibres, such as Zekrom, Rayquasa, Palkia and Reshiram. It's 130 Attack is also nothing to scoff at, allowing it to hit incredibly hard. And even though it is weak on the defences compared to Dragonite and other bulky pokes, it still have really solid HP and Defense, and not terrible Special Defense. And it even have decent Special Attack, granting it the capability to use its Special moves to the full potential.
Unlike its other Dragon-brethren, Garchomp have a quite shallow movepool, but only compared to behemoths such as Salamance and Hydreigon. It gets just the right moves to make itself stand out. Earthquake, Outrage, Dragon Claw, Stealth Rock, Hone Claws, Sword Dance, Subsitute, Flamethrower and Fire Blast make Garchomp a terrifying foe to face.
Garchomp is actually to good in certain aspects. Due to the power of its Sand Viel Ability, certain battle simulators such as Pokemon Showdown, Sand Viel Garhomp is banned from using. If it didn't have its still awesome DW ability Rough Skin, it is very likely Garchomp would be banished into the highest tier of Pokemon, Uber, having it to battle with the likes of Mewtwo, Arceus and Darkrai. Sand Viel is still very much viable to use ingame and in wifi battles though.
Garchomps solid Dragon/ Ground typing is perfect for a fast, offensive Pokemon. Very few Pokemons can safely counter or switch in to old Chompy. On the other hand, the typing allows Ice types such as Mamoswine, Weavile and Cloyster to send Garchomp running to the hills due to the 4x weakness. Of course, there is always Flamethrower and Fire Blast to melt any Ice foolishly trying to take down the Dragon of Doom.
There are so many good movesets Garchomp can use, and I haven't the time to write all of them down, so I will just post a link to the What is a good moveset for Garchomp? page here on Pokemon Database.
Hope it helped!