one single piece of gear is jusy too hard to choose, so I'd make an armor set called "Assassin's Shadow" it'd be the well preserved remains of an ancient assassin's gear from before the golden age. It'd be a battle torn black cloak. The chest piece would have a variety of daggers (all aesthetic) and poisons on it(poisons on a belt), with a sword on the back partially covered by the cloak. the helmet would actually be replaced by the assassin's mask(painted steel). A white mask with one eyehole as where the left eye would be, there is a burn mark. Where the mouth would be is the word "sleep" in Japanese. Makes some sense to be obtained from a Hive boss (you know, 'space zombies'). Advantages of this armor would be huge boost to stealth, large boost to melee (speed, damage, range, all or any). But disadvantages would be less defense than most legendary/exotic gear (from the gear being old) but not so much less that no one would use it. Obvious hunter gear.

A weapon I would put in Destiny would be a heavy sniper called "Dead Man's Hand". It'd be a heavily modified anti-tank rifle. It'd be painted black and have a large scope, as well as tally marks scratched into the left side of the gun by the previous owner. To the left of the tally marks would be a carving of the playing cards that make up the actual "dead man's hand". This being an anti-tank rifle would be very powerful, so to even it out you'd take enough damage from the recoil to make it so you only pull it out during the toughest boss fights.

MJOLNIR armour. It would be hard to get. 'Nuff said