If it's below freezing, then two socks, next to skin a tighter knit merino wool like smartwool, and then over that a much looser knit like a wigwam sock. The critical point isn't so much the socks but that you need a shoe or boot a few sizes larger than normal so that the socks don't compress. Even then, when cold enough, you may also want a chemical heating packet over the toes between the two socks.
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