I've seen some amazing supply boxes even while my team was winning. A molotov, 100 parts, melee weapon, ect.
This happens when you are the last man standing and you went for the supply box
you could also get crafter bombs, purchasable weapon's ammo and random powerful stuff to get you back in game
EDIT: i dont know if this applies in supply raid if all 3 team member are waiting to respawn while you grab the supply box.