Great lightweight wool crew!!! - Smartwool NTS Micro 150 Crew - Mens

Merino wool, in my opinion, is the ultimate base layer. Instantly warm, never itchy, breathes when you're active. I usually buy a size larger and never worry about machine washing and drying (on low to med heat). Smartwool makes a variety of weights for different activities (or lack of), so you can tailor your layering to what your plans are.