In real cold a pair of Sami reindeer hide boots, or home made snow moccasins (like the ones in Conovers book, or on RMs feet in Northern Wilderness). I have a pair of Empire Canvas Works mukluks on order, I hope to get them by winter. I have worn a pair of Nokian winter wellies (with a removable felt liner) before, but they died after many years; they vere fairly flexible, and I would not like a stiffer boot. I hear the Canadian forces mukluk is also quite flexible.
I have a pair of Sorrels, but they do not encourage movement of the foot, and my achilles tendons have grown sensitive to lashed up boots after a bad achilles tendon inflamation, even over 5 years later.