The easiest way to stay warm is to stay dry. No cotton.
Merino wool is my go-to option for a warm base layer. IceBreaker merino in various thicknesses keep me warm, whether I’m hiking backcountry after elk or sitting in the deer blind back home. A down vest mid-layer is my other must-have. I’m also happy with the micro-grid fleece mid-layers to help keep warmth against by body.