A durable, warm, breathable, thin ski sock is a hard thing to find, but the Rocky sock fits all of these categories perfectly.
Pros: The wool/synthetics blend combines all of the benefits of wool with all of the durability of synthetics. Fits like a dress sock and is easy to squeeze into my super tight boots. Plenty of padding, but only where it is needed (full shin, heal, ball of foot, and toes). Very breathable and keeps the feet dry, but they are still able retain some heat (I used these in -12F temps and had no problems). Extremely flexible, does not restrict movement or circulation and wont fall down into the boot.
Cons: Havent found one yet!
Overall: The best sock at an amazing price. Blows some $40 pairs out of the water.