Years ago I was out one winter at -13F, standing around chatting with people camping nearby while being perfectly warm and comfy. I used a large Marmot baffled down parka, plus the appropriate down pants and insulated boots.
Haven't had a chance to test drive my Nunatak Torre parka, but it seems like it should perform almost as well as the old Marmot.
Feathered Friends also has several expedition weight parkas, one particularly heavy (and expensive) one that would keep you warm in very cold conditions.
As others have mentioned, pair this with matching down pants, and don't forget *really* warm mitts - I'd suggest taking a look at the warmest that RBH Designs has, they have a built-in VBL layer inside, and it makes a huge difference.