Oddbean new post about login | logout best apocalyptic tool? my vote: bolt cutters
Cordless zipcut
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.