Garchomp hasn't gained a whole lot this Gen. He's got Rough Skin, making more useful in Non-SS teams, and also Dual Chop, which can break Subs.

SubChomp @ Lefties/Life Orb
Adamant (+Atk -SpA)/Jolly (+Spe -SpA) - Sand Veil (If in SS Team)/Rough Skin
252 Atk/252 Spe/4 HP

~ Substitute
~ Swords Dance
~ Earthquake
~ Dragon Claw/Dual Chop/Outrage

Substitute helps Garchomp set up Swords Dance, and protects him from WoW and other Status Problems, which can hinder his sweep. Swords Dance boosts his Attack for much harder hits. Earthquake is a powerful move that gets STAB and good coverage. Dragon Claw is a standard strength STAB. Dual Chop hits twice for 40 damage each, breaking substitutes, but at a cost of 10 less accuracy than Dragon Claw. Outrage is both powerful and accurate, but it locks Garchomp in confusion, and he's be unable to take advantage of openings to sub/set-up, so it's more useful after Garchomp has already set up one or two SD's.



BandChomp @ Choice Band
Jolly (+Spe -SpA) - Sand Veil (If in SS Team)/Rough Skin
252 Atk/252 Spe/4 HP

~ Earthquake
~ Dragon Claw/Outrage/Dual Chop
~ Fire Fang
~ Stone Edge/Crunch

Earthquake is the powerful STAB move that gets great coverage. Dragon Claw/Outrage/Dual Chop are all powerful Dragon STABs, each with their own advantages. Dragon Claw is a standard, Outrage is very powerful, but locks you in confusion, and Dual Chop breaks subs, but has 90 accuracy. Fire Fang destroys Ferrothorn and hits Skarmory, and Bronzong hard. Stone Edge gets superb coverage with Earthquake, but Crunch hits Psychics such as Cresselia hard.



Specs.Chomp @ Choice Specs.
Timid (+Spe -Atk) - Sand Veil (If in SS Team)/Rough Skin
252 SpA/252 Spe/4 HP

~ Dragon Pulse
~ Surf/Draco Meteor
~ Flamethrower
~ Earth Power

Dragon Pulse is a Gen.4-Only TM Move. It's SpecChomp's only viable Dragon STAB, aside from Draco Meteor. Draco Meteor is very powerful, but it hampers Chomps sweeping abilities. Flamethrower destroys Skarmory, Bronzong, and Ferrothorn. Earth Power is Chomp's powerful Ground STAB.

The main reason to go Spec over Band IMO is because of Skarmory's high defense that could tank a Fire Fang. It's usually not worth going Spec. over Phys. because Garchomp's Base Atk is much more impressive than his Base SpA.



Counters

Bronzong (116 Def) and Skarmory (140 Def) resist STAB Dragon Claw/Dual Chop/Outrage, and are immune to any Ground moves, making them viable switch-ins, and making Fire Fang all the more important. Cresselia is bulky enough to resist Garchomp's attacks, is immune to Ground attacks, and hits back with a powerful x4 Ice Beam.

Partners

Not sure about this one. Garchomp benefits greatly from Sandstorm because of Sand Veil's evasion boost, giving him more time to set-up and attack.