Bouncing this thread from April 'cos nobody mentioned Xerotherms!

That's what I use (under a neoprene drysuit) and I can't speak highly enough 
of it. I only use the thin one (not the Arctic) but with neoprene it's good 
for 10-11deg. Colder, you can put what you like on the top - or the Arctic. 
It's comfy, easy (top, bottom, socks). It's not at all bulky, folds up 
small, wicks like anything and works when wet.

If you do get one, don't believe any rumours about not washing them...

Bernard