It depends on the ability.
Dragonite and Garchomp are equally good, however, there's something that might set it apart.
If Dragonite has Multiscale, throw Garchomp into the trash, you don't need it anymore, Multiscale is absolutely broken, and I wouldn't even blame myself if I released every single Dratini into the wild due to not having Multiscale despite how cute they look.
However, if Dragonite just has inner focus, I would go with Garchomp, as Garchomp's dual STABs deal a lot more damage, especially for the elite 4, where you can just one shot 3/4 of all members of the elite 4 with Garchomp.
