Dark armor, easily. At low levels, DA benefits a great deal from Staff's endredux. At high levels with stable Endurance, it benefits a great deal from Form of the Body; which provides a combo-based damage buff to Death Shroud, an AoE -res, and a +res for self. Staff, meanwhile, loves the mez and damage auras; it's not the highest damage set natively and really wants to have a lot of people in its cones.
To a lesser extent, Fiery Aura, SR, and Electric Armor; FA is a lot like Dark here but a bit more frantic. The endredux doesn't help Fire quite as much (although it lets you take Blazing Aura earlier), but the +res really helps FA down the road. SR meanwhile gets an early Parry out of the deal and a +recov/+regen proc, doesn't hate the +res, and offers a +rech which Staff very much wants to get its combos out faster. Electric Armor combines a bit of both.
Hmmmm what do you think is better, Staff/Dark for scrappers or for brutes?