Simply the best
Western Mountaineering makes sleeping bags, and not much else. THIS is what they do they make the world's best sleeping bags.
If you want the absolute warmest bag on the planet, THIS is the bag to buy. Other manufacturers make "-40" bags, but no one packs as much down into their bags as Western Mountaineering. They cost a bit more, but you're paying for more down. More down = warmer bag.