I am a huge fan of merino wool. It is warm, soft, comfortable, and stylish. Not only that, but it is environmentally friendly! Only downside is the relatively high cost. I think all considered it is worth it, especially if you are careful to shop during sale times!
First item is my Smartwool, gray stripe s/s
This is a great, versatile piece and I am very happy with it, need to get more short sleeves for the slightly warmer months.
Ibex Zepher Zip - Ibex' thickest base layer. Very soft! pretty warm and stylish, although it is a little bit big for me as it is a size large. Certainly not a deal breaker. Purchased on Black Friday from the Ibex store on Newbury St, Boston.
Icebreaker Mondo Zip Tee - Bright red, a favorite of my wife. A very slim fitting piece, I think this has to do with the provenance of Icebreaker, people not from North America are just smaller!
Ibex Woolies - Thinner than the Zepher and more appropriate for wearing as a layering piece. I like the black and white stripe design. This is also the piece I sleep in most often!
Ibex Shak - given to me by my wife, its one of ATL Essential: Winter Edition because it is such a great piece! Much thicker, not a base layer in any but the most outlandish locations. This is an amazing mid layer and can even be worn as an outer layer on milder days. I tend to pair this with my Smartwool short sleeve.
I sometimes wear synthetics too. Patagonia Capilene 3 and a Regulator Zip fleece mid layer. Not as warm and tend to get stinky, but I got them awhile ago.