Craft W Active Crewneck LS
Craft came to fame on the strength of their baselayers. The Women's Active Crewneck Long Sleeve Base Layer is the core of that strength.
The key to its success was, and remains, the vertically-ribbed polyester fabric that created pockets of warm air and was able to both wick sweat and channel vapor for maximum effect. For these to work to their maximum, they have to fit like a second skin. Flatlock stitching means you won't notice the seams as they hug your body. The fabric now goes by the name Active, but it was once the proZero. It's good from 60 degrees Fahrenheit down as low as you can go.