I will speak up for choice scarf Mamoswine with Thick Fat purely because many electric types are prepared with Hidden Power Ice, and what with many electric types having high Special Attack you may find Garchomp suffering, particularly as electric types are faster.
I'm not arguing that Mamoswine is better than Garchomp, it's not, but in the scenario you're speaking of I'm a Mamoswine advocate. I am biased though because the wooly bully is one of my favourites.