Brain- you ain’t gonna survive anything if yours isn’t up to snuff. The world is different today, but remember that back in 1840 a man could go walkabout into the wilderness and live for a year with only what he could carry. That’s the mentality it takes and that’s still essentially the same stuff you need today.
A short list of what’s stocked and ready here from second-most-important downward:
Water- life ain’t gonna to last long without it. In house, vehicles, other places near and far. I know where the creeks, rivers, and shallow wells are here and in the directions I’ll go if I can’t stay here. I know where it hides in normal life like water heaters and toilet tanks and gutters and washing machine hoses. I can purify and carry it. I’ll have water aplenty no matter what.
Knives- kept sharp and several of them. The most useful tool ever invented and as good a close-in weapon as there ever was if you know how to use it that way. Gathers foods. Makes other useful things. Good trading material. As vital to life as:
Fire- again many ways to make them (even big ones). Also a weapon if used correctly. More vital to immediate life than water in cold weather. Makes light as long as you can fuel it. Knives can help you get materials to build them.
Arms- if you can’t keep it, then it’s not yours to start with and someone else will take it from you unless you can stop them from doing that. Have only what you can carry on hand; you don’t want to leave an armory behind for your opponents to use against you. Choose based on your need but must have at least one man-stopper-sized pistol and a full reload for it along with the expertise to use it well.
50’ of 3/8 or heavier Rope- not paracord which will not safely support a falling person’s weight. Whatever paracord does rope simply does better and stronger. Six essential knots will serve 99% of your purposes. Some nylon carpenter or braided mason’s string is handy too.
Tools- if you got these you can make whatever you need. Multi-tool and SAK is just the start, add a crescent wrench and vise-grips. Hatchet serves as a hammer and a weapon, small single-bit axe does the same if you prefer that. A short bow-saw blade and a couple hacksaw blades take almost no space, some heavy wire (tie wire for rebar aka ‘mechanic’s wire’) and a green bough make a frame for them. Duct tape a must, just too handy not to have. Full set of tools and likely-needed parts in every vehicle.
Shoes- choose based on your need but you may have to walk a long way to safety so these are vital, thus you always have some suitable ones on hand in case the ones you’re wearing get lost, damaged, aren’t suitable for purpose or whatever. Shoestrings have a number of uses and non-canvas shoes can carry liquids or loose stuff like a pocket does. May help someone else, no great loss if you have to toss them to gain space for other stuff. Rubber soles make smokier signal fires.
Food- you gotta eat, but you can go quite some time without that. And you can’t carry enough food to keep you going what with everything else you need to have so learn how to eat from the world around you- it won’t be delicious but it will keep you alive and you’ll need it if your stored foods aren’t available or run out. Urbanites can gather food from their different ‘land’ differently but that supply won’t last forever so learn the edible plants in your area. Salt is essential to life and works wonders on found foods. Most important is now: stop stuffing your face! Train your body to function on only what it needs and your food will go twice as far. I do physical work for a living and I eat less than 2K calories a day in two meals only. My body is efficient, strong, and healthy. I rarely feel hunger. I can easily go 2 days without eating without any loss of anything, mental or physical. If you eat like a pig now you will have to continue that or you’ll lose those vital mental and physical functions when you need them the most.
Shelter- your mylar ‘space blanket’ is worthless for this, Some heavy contractor grade garbage bags do better, make rain ponchos, impromptu seeping bags, carry stuff and with the duct tape makes a much sturdier shelter in storms. A real blanket, preferably wool, makes a myriad of things from clothing to lashing strips to wicks to a sediment filter for water.
Medical supplies- again based on needs but at least enough to deal with the emergencies you can handle alone. One almost always missing vital component is a petrolatum bandage for sucking chest wounds; collapse a lung and you’re dead. Army surplus has those. Keep up-to-date, most medical stuff has a limited lifespan and likes cool dry storage. Longer terms need antibiotics and powerful pain meds. No good if you don’t know how to use the stuff you’ve got so learn that too.
Apologies for not being specific but my stocking patterns change seasonally and everyone’s needs are different so what works best here might not there. Lights fit in here somewhere, but there were no flashlights through most of human history so you can probably do fine without them. One thread goes throughout all of this- you must test your stuff abusively to know it is good enough, you must know how to use your stuff well and in every circumstance you might encounter, and you must practice with your stuff. That’s why the first item comes first; your brain used properly is the most powerful weapon and survival tool in the world. Know your stuff and you’ll be OK. Lack that and you won’t no matter how much other stuff you have.
Phil