Wool or synthetic long underwear
I've got military issue cotton and polypropylene so far.
I'm looking for something light-weight to replace the cotton for wear on a daily basis in the mountains that won't lose it's insulation properties if it gets wet.
I started out last week with just pants and layering on top (t-shirt, polypro shirt and flannel shirt or insulated jacket), that normally works just fine for me at lower elevations until it gets into the 30s.
I was up at 8600' - 9500', it was in the 50s-60s during the day, but I started realizing that it was a bit damp, the kind of stuff where hypothermia creeps up on you.
Next year, I'm going to need to wear underwear from day 1.