I've got three words for ya. Wool. Wool. Wool. No synthetic material matches for warmth or durability. My work wardrobe is all second hand, so though wool can get spendy, I can't remember when I've paid more than $10 for a good sweater. I do wear cotton thermal long johns. Mine stay on inside or outside from Nov-Mar.
Pack boots are really important when the temps get below 0*. Sorel, Kamik, or L.L. Bean all make good pack boots. LL Bean is still made in the US I think, and Kamik is still Canadian. Sorel now makes their stuff overseas.
I really dislike thinsulate and goretex. You sweat in them and then you freeze. And polar fleece is junk and it doesn't even make good shop rags.
There are also degrees of comfort. It's important to remember that if it's 20 below, you'll never really be comfortable. And if the windchil is -20, you're going to be miserable however warm you are dressed.