Mammut Kula Pull Shirt

This shirt is amazing! I went on a five-day winter mountaineering camp and wore only this shirt under my 3-1 winter jacket and never got cold even while others were constantly shivering. It can't be beaten for warmth or comfort. It's also great by itself when you've warmed up and taken off your jacket.

My one negative note would be that it seems to collect smells and while others may not smell you since it's well designed to hold them in, they never quite seem to wash out.