Smartwool socks.

I discovered them downhill skiing and have been using them every since. Thin yet warm, I put a very thin pair on first and then a thick pair (make sure you have room in your boots). Too tight boots are a major cause of cold feet when I worked in the rental shop so that should be #1. I too believe in breathing clothing so I ONLY use boots with Gore-Tex (have yet to find an alternative that works as well as genuine Gore-Tex).