The right base layer for you is going to be the one that keeps you dry by moving your body heat and sweat away from you. Which base layer is best at this depends on what you're doing. Well, then you might need two different base layers. At the same time the very lightweight merino layers, while great for high-intensity activities, aren't as warm when you're not moving. The Carhartt synthetic layers stand up better to wear than merino (even, often, merino blends) and cost less.