Right so. You told me this is doubles. All the other answers seem to pertain to Singles rather than Doubles. Lol dammit.
Bisharp, Scizor, Genesect, Aegislash
All four of those are good offensive Pokemon, with the ability to deal with Fairy and Ice types with their STAB and also resist Dragon type attacks (Those are Chomp's 3 main weaknesses).
>Steel-types such as Scizor, Genesect, Bisharp, and Aegislash are excellent partners, as all 4 have nearly perfect defensive synergy with Garchomp, can defeat the Fairy-, Dragon-, and Ice-types that give Garchomp trouble, and are able to defeat Cresselia with their secondary STAB moves. Bisharp stands out as a partner, as it deters Pokemon with the Intimidate ability from switching in with its own ability, Defiant. In return, Garchomp is able to defeat the Fire-type Pokemon that give these Steel-types trouble.
Wow KoD is lazy!
Thundurus, Togekiss, Zapdos, Cresselia
All of those guys are immune to Earthquake - also, they provide valuable Speed control in the form of Thunder Wave and Icy Wind
>Good partners that can provide Garchomp with speed control include Togekiss, Zapdos, Cresselia, and Thundurus, as many of these Pokemon are immune to Garchomp's Earthquake- Flying-types and Pokemon with the ability Levitate are great partners in general, as they are immune to Garchomp's Earthquake, allowing Garchomp to freely use the move without any drawbacks
Smogon Dex (Doubles) o3o