I don't think there is a single base layer that is "best" for every condition. Layering to suit conditions is the key. My experiences below address layering under a gore tex or e-vent shell.
Next-to-skin I wear a washable silk turtle neck as a base layer. I have used Teramar, Lands End, Cabellas, whatever brand is on sale cheap (check STP). I don't see much difference between the silk t-neck brands, although some claim to be thicker than the others. Don't expect the silk to last more than 2-3 winters and pick dark colors (looks new first winter of use, looks worn by the second winter of use, has small holes by the end of the 3rd winter).
On top of the silk t-neck I usually wear a merino crew top. My favorite merino top is the Ibex ribbies (thicker version of the Ibex woolly). Unfortunately, Ibex hasn't made the ribby for a couple of years (I haven't seen it available on their web site). When they make it again, I will pounce. The ribby is not one of the new, super-fine 18.0 micron or 17.0 micron tops, so it is a bit itchy if it is directly against the skin IMO.
Over the merino, I wear zero-to-two layers of fleece, depending on how cold it is. When two layers of fleece are needed, the bottom layer is a polartec powerstretch top because it layers under other fleece very nicely.
For my legs, I have never needed more than a pair of polartec power stretch bottoms + my goretex shell ski pants to stay warm. My feet get cold, but not my legs, not even when the temps are 10 to 15 degrees below zero on the fahrenheit scale. What I really mean by that is, my legs never get cold enough to force me into the lodge. When I get too cold to continue skiing it is almost always due to cold feet, hands or face.