The warmest socks that I've encountered are RBH vapor barrier socks.
http://www.rbhdesigns.com/product/34/vaprthrm-insulated-sock.htm
Since the vapor barrier is bonded together with the other layers, it provides much more secure footing relative to using slick plastic bags, as it should considering the cost. And of course they are also vastly more durable.

I have trouble with cold feet. I have solved this with this combination...when winter camping I wear thin, wool liner socks that are not tight, thick, ragg wool socks inside Sierra Down booties that have Toasty Feet Aerogel inserts. I also make sure my legs are well insulated and protected with windproof pants...Smartwool base layer, Montbell Inner down pants, either REI Element pants or Pre-cips on the outside...hat to keep my feet warm...

heated water bottle inside the bag...it's a good thing...

I've used the vapor barrier principle since the early 70's. It's fine with two exceptions: Potential "fungus feet" if used for prolonged periods and foot rotation within footwear when perpendicular to a slope.

I use a polypropylene inner sock, then a plastic bag and then Acorn fleece socks. In very cold weather, particularly when snow would cause heat loss through conduction I put chemical heaters, (140 deg. F)under my toes in the toe box of my hiking shoes. I store chemical heaters as a survival backup as well.

A lot of good info here. I use combinations of what is mentioned.
I especially appreciate the point of not constricting the feet. This can happen when you start putting thicker and more layers of socks in your shoes.
I do 99% of my backpacking in sandals all 4 seasons. This is what I appreciate about Tevas (TerraFI2, 10oz ea). I have enough extra Velcro strap that I can don a coolmax sock, thin wool sock, and then a Sierra Design Bootie (3.1oz each) w/o compressing the bootie. It is very comfy and warm. The sandal insulates everything from the cold earth.

I tried a fleece sock layer but went back to wool since it’s warmer (for me).

May everyone spoil their feet :)

-Barry

Again the Feathered Friends Down Booties.

I bought a pair of the Nunatak Chugach booties here on the gear exchange. The may be a little lighter and possibly a little warmer than the FF Booties but not as versatile. The FF booties has the inner bootie and an outer Epic shell. I could tromp around outside in the snow and then remove the outer shell and wear the inners in the tent. I could sit in the snow cooking with the FF booties on in 20 temps and my feet were cozy, wearing only liner socks.

I live in Texas so I don't need to pairs of down booties so if anyone needs XL Chugach booties IM me.