For retro's sake, the Rocket Launcher gun upgrade from the original Bionic Commando. To be honest, it probably was a little overpowered, but its high power and ability to fire through enemies was just too awesome at the time.
only in the first LoZ do i count the master sword a power up, and all other wepons because the only thing that you have to get other wise you cant move on in the first LoZ for the nes is the wooden sword
p.s. they should bring sword beam back
If everyone who followed their dreams made it, we all would be famous.