For the energy drain/corrosive touch reasons mentioned in the gestalt/citybot thread, I'll say Monstructor. Galvatron, power-wise, will be a tall order to deal with, but still I pick Monstro.
That would depend on the fact whether the fight would take a place after or before destruction of Unicron, since Galvatron receives unlimited power from unicron himself.If not, then Galvy would call all Decepticons to help himself, there fore destroying Monstructor completelly.