Long sleeve thermals in lots of different sizes are cheap and easy to layer for perfect warmth. I have a bunch of Level 1 thermals I got for a great deal. Down is expensive and a no no in wet conditions, not worth the hassle when you're skiing in my opinion. Good waterproof outer layer, cheap comfortable thermals, dri fit tee shirt on the bottom for the win.
For me, it's dependent on weather. Really cold days:Under armour base layer, rei quarter zip fleece, primaloft jacket, saga shutout. 20-30ish: Under armour base layer, rei quarter zip, hoodie, saga shutout. Above 30: Base layer, flannel, my primaloft jacket, but vested.