Integral Designs XPD3
The XPD3 has been discontinued. If you're looking for something new, check out the best cold weather down sleeping bags for 2020.
I tried so much winter bags! Now I found mine! This bag is really well made and light for its warmth. The Pertex shell really breathes well. The best thing with this bag is the "kind of" snug fit it provides. You can really feel that there is no unwanted air pocket, but the bag still very comfortable. I tried it often in very cold temperature and it kept me warm even when I wasn't sure if it would (only in a bivouac on a -40F night with high winds). I would trust my life on it!
Fill: New one is 750+ and old one is 800+
Temperature Rating: -40F
Weight: About 5 pounds