I generally just use regular, old polypropylene. I do have some of the fancier things like Coolmax-based layers, but I can't tell the difference. I'd rather have my base layer keep me dry than insulate. I'd also rather have 10 good sets than 1 really nice one. El-cheapo polypro from somewhere like www.sierratradingpost.com fits the bill.
I love patagonia stuff. I have some silkweight, lightweight and this week I bought some new variable-something midweight stuff. It lasts forever. I have a zip neck ligthweight top that I got in 97, and it still looks like new.
I love silk as a first layer and if colder add a polypro mid weight and in extreme condition(really cold go with expedition weight poly) I support REI since the early 70's. You get dividends and they test their product. Very good customer relations.http://rei.com
Icebreaker - the best easily...... last for years NEVER smell - even if you wear them for a week without washing (No! I haven't tried it - but a friend has a guiding husband & she says he is allowed in the door on return from a week backcountry now)
I like the Patagucci silkweight capilene shirts and pant/bottoms. They're available through REI (will second the mention above). These rerely if ever go on sale anywhere, so buying from REI is a good deal.