Many years ago Extremities Mountain Toesters were the first merino wool socks I tried. I was impressed and I still am. These are the thickest, warmest socks tested and ideal for winter, especially combined with heavy leather or plastic boots. I use them with my plastic ski touring boots and find them very comfortable. As well as warmth the thick cushioning provides more protection against the stiff materials of winter boots. The cushioning is thicker on the sole, toe and ankle. To ensure a secure fit there is elastication at the ankle and arch. The long leg is only cushioned at the front, the rear being plain knit. The socks are knee length and there’s an elasticated cuff at the top so they stay up.  

These aren’t the lightest socks and the price is higher than many but for maximum warmth these are the ones to choose. If you don’t want them knee length there is a Short version that costs £20.