Wool for the win
I have three different thin balaclavas, and this is probably my favorite. I typically wear them for winter bike commuting, but they all pull double duty on the trail. One is a stretchy standard polyester Mountain Hardware Power Stretch and the second is an Outdoor Research Sonic with mesh over the mouth and a nose hood. The Smartwool is easily the most comfortable of the three; it's stretchy enough to fit, but doesn't smoosh my nose like the MH. It also breaths a lot better than the MH, which is most noticeable when your glasses or goggles are fogging up. The OR keeps eyewear from fogging up the best, but at the expense of cold lips and being somewhat uncomfortable because of it's design. The Smartwool is also flexible enough to be worn with my face completely uncovered. The MH being less breathable and polyester also starts to smell after just a couple uses around the face, the Smartwool does not. I think it's so breathable and comfortable because it is the thinnest, but it doesn't sacrifice any warmth, because surprisingly it's easily as warm as the other two, even with the wind of riding. For scale, I typically wore this (and the other two) between 35F and 5F actual temp, swapping to something much thicker below 0F.
The one negative is that the Smartwool, by virtue of the fabric, doesn't rebound from being stretched as well as a synthetic fabric. After a couple uses the face area below the "hem" around the opening can get slightly stretched out, especially if I'm pulling it down over my face a lot. A wash restores the shape.