When the weather here turns nasty I don a pair of Cabela's bibs with fleece pants under that and next to the skin I wear "Smartwool" pants, which is basically Merino wool and it doesn't itch. I layer the upper part so to be able to adjust my clothes. Layering especially important if you will be walking. If stand hunting insulated boots large enough to accommodate thick socks are important. Kevin made a good point about not having boots laced (fitted) too tight, doing so will result in cold feet no matter how good the insulation in the boot is. Also warm headgear is very important.
P.S. No cotton.