Polypropylene is one of the most thermally AND water resistant (warmest and driest) and yet lightest fabrics commonly used. However, it's weaker than most synthetics and from what i have heard, stinks to high heavens. I don't own and haven't used these at all so i am not speaking from experience on that aspect. I'm a relative youngin and PP is not as common or popular now as it was.
As far as thermal resistance goes, next in line is silk, then polyester not far behind, then wool and acrylic about equal, then nylon, hemp, linen, tencel (the "nano fibril" effect makes it warmer than some of it's cellulose cousins), and then cotton. But there is more to warmth than the innate thermal resistance of the fiber matter itself. There is the shape and structure of the fiber, the diameter size, how it's weaved, etc, etc. Much of the latter relates to how much air it traps. Generally speaking, the finer and more hollow the fiber the warmer it will be, but the scales on wool also helps. Completely round and solid fibers aren't as good as trapping air. Trilobal fibers like silk trap more air than round, etc, etc.
But baselayers aren't used so much for warmth unless you're talking real cold, they are primarily for moving sweat, staying dry, and comfort.
If silk is treated to make it more hydrophobic, then it's not a bad choice in holistic terms of lightness, warmth, stink control, and strength (while it's not the most durable fiber around, it is quite strong for a natural fiber [high tensile strength], but personally i think it's better for a layer over the baselayer for colder weather and high wool blends better for next to skin).
If i recall correctly, there was a field research study done awhile back to see if what one of the original Euro mountaineers who attempted to summit Mt. Everest wore, would be feasible and realistic to do (this particular historical fellow did not make it to the top). I believe i remember reading that he wore something like 7 or 8 alternating layers of wool and silk. What was surprising in the study was that not only was the researcher(s) able to take such temps and conditions in this archaic get up, but it was actually comfortable.
In a lot of ways, nature is still yet wiser and more advanced than man. Especially in the balance area.