I will preface by saying I play and work in extreme cold. Deer hunting, drill rig, ice fishing...all relatively sedentary activities, done to sub zero temps.
For socks, I wear silk undersocks. They are warm, thin, and slick. Properly fitted they won't constrict your circulation which is vital in cold weather. Over this, down to about the teens or so, I wear a heavyweight Merino wool sock, Wigwam brand. Other brands are of similar warmth and quality, but it is important they are merino. They are warm for their bulk...also not constricting your circulation. Dressed accordingly, my feet will stay warm in uninsulated work boots to the mid teens, even when damp/wet.
Now for real cold. I still wear the silks. Now my oversock is changed to an Alpaca wool heavy sock made by Alpacas of Montana. They were expensive and worth every penny. My boot liners also contain some neoprene and space age reflective material. There's only so much conventional pack, muck, rubber insulated boots will do. When it gets below zero, you need some specialized boots. Mine are Viking brand, as my boots must be electrically rated. I use them for hunting and such also. Other quality extreme cold boots are made by Baffin, LaCrosse and others. I will also pack an old sleeping bag and put my feet and legs in it in the stand. This helps tremendously for additional warmth.
