My 25-y-o LLBean Baxter State (Park in Maine) parka was rated to -40 and that seems right (as part of a clothing system). Its worked for me, plenty well enough at -25F vans -30F. I’ve been in -40 twice but screw work!, one does the bare minimum outside and gets back to a vehicle or building.
It’s got all the features, doodahs and fill weight. And a thick, down-insulated tunnel hood with a fake fur ruff, etc. I’ve been going through my winter gear and replacing the fake fur with real fox or wolverine on the ruff.  It’s impressive how much better it slows the wind.  Like it had evolved to do so over 50 million years!  The current model, by price and specs is clearly much less insulating.
I’ve also got the Patagonia “Belay Parka”. $700 I think it was, with crazy high loft, low weight, and lifetime cleaning included. I’ve only used it as emergency equipment heading to Prudhoe Bay or similar in winter.  The outer fabric is so light, I don’t want to risk it around a drill rig or bashing through the spruce.