Ibex Woolies Pro Tech Crew and matching Woolies Pro Tech Bottoms ($115) are the best lightweight merino base layer I've tested. They've been the top pick in our merino wool clothing guide for years, and I struggle to imagine what would unseat them. Woolies are Nuyarn, which is 85 percent merino wool and 15 percent nylon. The merino wool is wrapped around a nylon core, which increases the warmth while being lighter (5 ounces for the men's large) and more durable. I've been wearing Woolies as my main base layer in winter for over three years now.