My weapon would probably be a silenced custom sniper with the ability to toggle active camo (Think MGS4 Snakes Suit.) that shoots armor piercing explosive rounds, has infinite ammo and it would take no maintenance, I would call it "Ghost" and the bullets would all have the jolly roger etched into them. And it would have a dagger that you could pull out of the stock, The dagger would be blackened steel and have a snake etched into it on both sides of the blade.
OR a tree branch that grows money. (I would call that one "Cha-ching") But in all fairness I don't really count this one since I would probably never hit anyone with it...