For maximum warmth per ounce, reviews say down sleeping bags can't be beat. Good-quality down also keeps its loft a long time, and down sleeping bags feel especially soft. Quality is measured by fill power – an ounce of 800-fill down expands to fill 800 cubic inches, so it has more insulating power than 600-fill down. Since down loses much of its insulating power if it gets wet, down sleeping bags usually come with water-repellent shells.
Two 20-degree mummy-style sleeping bags vie for top honors as the best down sleeping bag: the Western Mountaineering UltraLite (*Est. $360) and the Marmot Helium. Both use 850+ goose down for extra warmth with minimal weight, and share top ranking in a very detailed, thorough comparison review of 11 high-end sleeping bags at Backpacking.net. Construction quality is judged excellent, and both sleeping bags have big draft tubes to keep cold air from leaking in at the zipper. The outer shell on both is Pertex Quantum, which reviews find durable, water-resistant and reasonably breathable.
The Western Mountaineering UltraLite (*Est. $360), top-ranked at Climbing.com, is very light at 1 lb. 10 oz. for the middle length. The five-inch loft provides plenty of warmth, and the sleeping bag has a two-way full-length zipper – nice for good ventilation and for pairing two bags to make a double. Owners reviewing it at BackpackGearTest.org give it rave reviews after long tests.
The 20-degree Marmot Helium is five ounces heavier, but earns the 2008 Editor's Choice Gold Award at Backpacker Magazine. According to editors, "Each bag in this line is overstuffed with the highest quality feathers we've seen...;.well-distributed throughout." It's also one of five sleeping bags recommended at Outdoor Life, after tests in a zero-degree freezer confirm that it's plenty warm. The draft tube is big, and the bag's seams are on the bottom where a sleeping pad will insulate them from drafts.
The less expensive 15-degree Marmot Pinnacle earns top marks in owner-written reviews at REI.com and Trailspace.com, though it weighs more at 2 lbs. 8 oz. -- still not too heavy. Owners say it's warm, water-repellent and very comfortable. The 15-degree Marmot Sawtooth isn't as water-repellent and weighs almost three pounds because it uses only 600-fill down. At this price range and weight, consider The North Face Cat's Meow (Est. $170 and up), since its synthetic fill provides warmth even when wet.
For heavier individuals, reviews often recommend Big Agnes sleeping bags, which minimize weight by slipping a sleeping pad into a pocket underneath instead of insulating the bag there. Down underneath a sleeper compresses a lot anyway, and this system also keeps the bag from slipping off the sleeping pad. Outside Magazine's 2008 summer Buyer's Guide recommends the 1 lb. 14 oz. Big Agnes Zirkel SL 20, which uses 800-fill down and has a full-length zipper. The bag has nice features like a built-in pillow pocket plus internal ties for a sleeping bag liner.
The Big Agnes system is good if you like to sleep mostly on your back, but if you toss and turn a lot, reviews recommend MontBell Super Stretch Down Hugger sleeping bags. Each baffle has elastic so the bag changes shape as you move around, and owners say they don't feel constricted. These mummy bags have accumulated other good reviews over the years, and come in a wide range of temperature ratings. The 15-degree MontBell SS Down Hugger #1 weighs 2 lbs. 4 oz.