For extreme cold weather I'd go for these
http://www.mgear.com/pages/product/p.../0/item/103676
Anyway's these boots, are good for extreme mountaineering, crampon compatible, plus they have a liner, which if it gets wet, you can pull it out of your boot, and put it in your sleeping bag, to dry. Then the next morning, knock the ice off the outer boot, and put on your dry liners. But they are pretty pricey, so only worth it, if you are going to use them alot.