Other contenders:

Sillcock key

Water filter

Mylar blanket

Fresnel lens

Knife

Shotgun/Rifle

Tourniquet

Pry-bar

CB Radio

Paracord

I think there's basically two ways to look at the question: bottom-up (starting with most immediate needs like air, warmth, water, wound care and food) or looking top-down at the tools that provide the greatest potential to meet many needs like a gun, an entry tool, communication tools, etc.

I think overall the single most powerful item is probably a radio.