Snowshoeing- polartec powerdry
Downhill skiing- expedition weight fleece

I use Polypro,

I've used cotton, silk, polypro, and wool.

Hands down, silk wins. Cotton is useless after about a day.

Polypro stinks after a day, and melts when you're next to a fire. Washing it to remove the smell only makes it smaller.

Wool would be fine, but it's thick and makes me itch.

Silk works.

Long Johns are the transpiration layer to wick chilling perspiration off the skin ( unless you buy into vapour barrier lining theory.) As such, cotton actually works well if you are garanteed 20 degrees or lower. Anything above and it slowly begins to earn it's 'cotton kills' reputation. Another option are fishnets, invented in Scandinavia and available from Brynje and Wiggys. The theory is the netting allows VERY rapid moisture transfer and traps larger pockets of warm air in that initial layer. Longjohns also come in various wieghts for different temperature ranges. There is no reason not to try and even carry more than one set for true versatility.

Only experience I have had is with either the old-fashioned cotton thermals and polypro. The cotton worked fine until it became the least bit damp, then it was just cold and uncomfortable.

Poly-pro seems to work well with 2 cautions, after a couple of days it really begins to stink and it seems to loose it's effectiveness. I usually pack at least 3 pairs for cold weather: one for sleeping, one for wearing, and one to be rinsed out.

I have switched to wool for the most part. Smartwool to be specific, longs are excellent. They are super soft and non-itchy and handle moisture very well. I have almost completely abandoned my capiliene in favor of these and have absolutely done so in dry cold. I have two weights of Smartwool bottoms and both are superior in every way to any plastic underwear I have ever used.

Byrnje poly string is excellent for high mositure activities like cross-country skiing, and combined with Smartwool on top of them it is a super combo for very cold.

The wool tops I use are: Icebreaker, Smartwool and Filson. The first two are much softer wool than the Filson which I only use as an outer layer top.

Just a note to second what you wrote about Smartwool.

One misconception about wool is that it is scratchy. That is true in some cases, but not always. Not all wool is created equal.

Different breeds of sheep produce wool fibers that are different lengths and diameters. The price of raw wool is based to a large degree on fiber diameter. Small fiber diameter is best for high quality, soft materials. Course (cheap) fleeces go to applications like carpet and rugs.

The Merino breed produces a fine diameter fiber. Smartwool is Merino wool.

I once had a wool army overcoat that would sand paper my neck every time I wore it. Rag wool sweaters are course and scratchy. The wool US army blanket is made of courser wool. Some cheaper XYZ-Mart undergarments use cheap wool. Cheap wool socks can also give wool a bad rap.

But, good quality wool undergarments ($$) can be very soft if they are made with the right kind of wool. I just spent the weekend camping. My Smartwool hiking socks were a pure pleasure.

I've been wearing silk longjohns for years. Very comfortable, lightweight. Used to buy them from LL Bean, not sure where the wife get mine now.
Cleaning is PITA, hand washing though.

For active wear I put wool undies over Wiggy's fishnet. Heaven!

Get the best grade wool you can find - Smartwool OK - fine gauge Merino wool from NZ - better. Make sure you get the thinner. NFL teams wear it at Lambeau - 'nuff said!

You may want to look at some of these products:
REVELATION A/M™ ANTI-MICROBIAL

MEDALIST® SILVERMAX™ TACTICAL SHIELD UNDERWEAR

There is also this product, which it looks like there are not selling to the public at the reduced price, they might at regular list price (you would need to check):
GEAR FOR A WARRIOR™

Pete