Icebreaker did good work with the Sprint Crew. This baselayer is light but warm and even has nifty thumb-holes for additional comfort. It beat out fierce competition during field testing to win our Editors' Choice award. While other baselayers may be softer, cheaper, or more breathable, the Icebreaker Sprint Crew is what we grab when the weather looks temperamental. We use it for everything, from whitewater kayaking to big wall climbing to wolf tracking.
This baselayer, while comfortable, is designed for movement. For something a bit more cozy, try the Ibex Zepher [sic]. If the $100 price tag for the Sprint Crew seems a bit steep, go synthetic with the Patagonia Capilene 2 or 3.