Summer and winter are completely different beasts. The type of boots you have and whether your feet are prone to getting cold are also factors.

In the winter I personally wear a pair of poly liner socks and then a pair of wool socks over that. Wool is a very good insulator and continues to insulate when wet. My feet are prone to getting cold, so I like thick socks in the winter. I also wear Sorel Conquest Boots which provide a good degree of insulation. In the winter you do not want so much sock that your boots feel tight as that will cut of circulation and result in cold feet.

In the summer I normally only hike in a single pair of lighter synthetic socks. I normally wear trail running shoes as I like to pack light and move fast. If your feet are prone to blisters than a thin poly liner sock plus an outer synthetic sock might be a good idea.

The bottom line is to stay away from cotton.