Capilene 3 Zip
Patagonia makes some fantastic baselayers in the Capilene 1,2,3 and 4 versions. This is the midweight zip version and is a comfortable shirt to wear, looks good and does a great job of wicking moisture. The zipper is a nice option to vent if you start getting a little warm. I have noticed a significant difference in warmth between this shirt and the Capilene 1 and 2.
Patagonia considers this a slim fit but I think of it more as an athletic fit, fits snug but not tight. It is not compression fit like some other baselayers.
In most conditions, this baselayer and a vest under a good shell is a great combination.