After doing research from various high end brands. These Castelli Men's Polare 2 Bibtights seem to be regarded as the best bibs for the coldest weather.
They are warm enough in fact that I would not recommend riding with them in temps over 40F. I rode in some 25-30F weather with a good deal of wind recently. And my legs were doing just fine.
As with any Castelli product, if you are an American you really do need to size up. And these bibs are definitely no exception. Because they are so good at being wind proof. They do not really stretch much. So, you won't be able to shove your legs into these if you picked the wrong size. I typically wear a large bib, and sized up to XL and they are nice and tight.
If you want to be able to ride in very cold weather, these bibs are really the way to go.