Really, for an extremely warm piece, any expedition-quality down jacket should be fine. An example would be Marmot's Pabat with a Membrain shell ($299). That's an extremely warm jacket. As is The North Face's Summit Jacket, also $299. Both use high quality 700-fill down, in a fill quantity that ought to be just right for anything close to zero degrees.
Still, I'll grant you those are cruiserweights. If you want the warmest, most serious down parka on the planet, then the choice is pretty clear: Feathered Friends' Rock & Ice. This is to down jackets what Ali is to boxing, the M1 tank to armored warfare, Hemingway to writers. It has 24 ounces of 700-fill down, double zippers behind a down-filled flap, two big inside pockets plus two big outside pockets (the day I summitted Denali, I didn't take a pack, I just stuffed my Rock & Ice pockets with spare mittens, water bottles, and Snickers bars), and a waterproof shell. This isn't a jacket, it's your own thermal atmosphere.
Alas, the price. Oh, the price. Such a price. It's $650. But man, if you get cold, this is the jacket for you.