I choose from one of several depending on the temperature. I like 1/4 to 1/2 zip base shirts with a high neck.
* HH Lifa Active for the fall and winter days warmer than -10C
* UA Base 2.0 1/4 zip for -10 to -20C
* UA Base 4.0 for colder (unfortunately no 1/4 zip option)

I tried a UA Coldgear shirt and absolutely hated it. A 2 layer sponge that saturates and left me freezing. Literally had ice forming on it.

I'm not a fan of merino as an upper body base layer. Certain weaves hold onto a lot of moisture, while others have a clammy greasy feeling.