I've ridden down to about 12 degrees F with:

Long sleeve baselayer (underarmour)
Cotton Tshirt
Wool Sweater
Windbreaker
Balaclava and helmet

Long underwear pants (cotton)
Jeans
Smartwool socks (base)
Crew socks (above wool)

My problem comes in with gloves...I have been through 3 pairs and none of them keep my fingers warm enough. The newest pair keep my palms, thumbs, and back of my hands warm, but the fingers themselves freeze. I have heard that Pipe gloves are good...but expensive.

I probably could have gotten by without the tshirt, but it was just a safety thing...