Actually now that I am thinking about this a bit more, I think some good tools tools and knowledge to help you survive are more important than one piece of equipment. If its -10 and any bag gets wet, your tarp or tent blows away or have any other of a myriad of problems, you should still have some backup tools and skills IMO.
I would learn how to build a debris shelter and sleep in it one night with nothing but a wool blanket or a med emer survival bivy bag and maybe in the rain. You can carry a space blanket in your pocket. The med emer bivy is a bit more refined and heavy duty but 7 oz.
Learn how to get your shelter working with a fire to help keep you warm, IE reflectors etc. That will take care of shelter and if you have a tarp bag or tent that just makes it easier.
With a good shelter, a good fire and some skills you can survive a very cold night in a wool blanket.
Maybe try a snow cave in the winter.
I have 3 survival items I carry. 2 all the time.
One on a Keyring, one in a small altoids can my pocket, and a suplemental pouch.
On the keyring, Fire steel and striker, GI can opener, Tiny flashlight, Tiny swiss army knife.
In a tiny altoids can I carry, $, 1 micro pack antibiotic
1 condom, 2 water tablets, 16' of spiderwire cord, Emer Saw, length of SS wire, Brunton button compass, 2 fish hooks, 1 needle, 1 xacto blade, 2 Suture strip bandaids, 5 wp matches, 1 micro mag glass, 1 length of tinder cord with a fat rubber band around the box
Weighs something like 1.5 oz.
In the suplemental I carry an emer poncho, 1 gal plastic bag, Larger piece of fire starter, gorrilla tape, 1/2 of a frostking sliding glass door insul kit (which is polycro about 4'x8' and very light),Space blanket, a few more bandaids, small gauze pad, a bit of toilet paper, tealight candle, micro flashlight, lighter, wp matches, a spare compass, iodine pills, micro bug spray, and a sport size quick clot. Those 3 go with me anytime I am outdoors hunting, hiking or whatever.
I also carry the swiss army knife with the saw most all the time. I am confident I could survive pretty much anywhere except severe cold for a day or two with that gear and some ingenuity.
Also when just out wandering around I like to carry a good knife, a gerber collapsable saw (the 3 oz one), a healthy length of paracord, gloves, bandana, and a campmor 5'x9' poncho/tarp. Also something to boil water in.