The best possible sleeping bag for backpacking is a hard idea to get an exact definition of, but with this list I've tried to go by a few key things like weight, reputation, reviews, and features. I've also split things up in to a few different categories to help you find the ideal sleeping bag.
Best Winter Sleeping Bags
Western Mountaineering Bison GWS - Western Mountaineering is known for making excellent, lightweight products, and though the Bison GWS is rated down to -40 Fahrenheit (and Celsius too) it still manages to be one of the lighter -40 sleeping bags on the market.
Mont Bell Super Spiral Down EXP - Mont Bell is a Japanese manufacturer that sticks mainly to producing down products, both sleeping bags and clothing, and they make some of the better sleeping bags on the market. The Super Spiral Down EXP has one of their lowest temperature ratings, coming in at a temperature rating of -20 Fahrenheit. This sleeping bag combines a lot of the innovative features you often see from Mont Bell to be seriously stand out lightweight sleeping bag.
Sea to Summit AP III - The AP III isn't as warm as the other sleeping bags in this category, being rated down only to -4 Fahrenheit, but it's a very well designed sleeping bag, and the 850+ down fill means you save on weight, so if you're not planning on needing the extra insulation of a warmer winter sleeping bag then the AP III would be an ideal choice.
Best Summer Sleeping Bags
Mont Bell U.L. Super Spiral Down #5 - An 800+ down fill bag good down to 40 Fahrenheit, so that you get all the warmth necessary without any of the extra weight that comes with the lower quality down normally found in summer sleeping bags. The U.L. Super Spiral Down #5 also has many of the innovative features you typically see on Mont Bell's sleeping bags, making it an excellent, lightweight choice for a summer sleeping bag.
Western Mountaineering HighLite - The Highlite is another very light summer sleeping bag from Western Mountaineering, who make some of the best sleeping bags you'll find anywhere. It uses 850+ fill down, and features box baffling to keep the down from moving around too much as well as the stand out purple fabric. The Highlite will keep you comfortable down to 35 Fahrenheit.
Sea to Summit: Mc II - You can tell right away by looking at the MC II that it's probably not for everyone. The thin design means that bigger people would probably be more than a little snug zipped up in it. But for the slimmer people out there the streamlined design, the 850+ fill down and the 36 Fahrenheit temperature rating and the ability to convert it to a quilt on the fly will make it an ideal choice for saving weight while summer backpacking.
Best Quilt Sleeping Bags
Nunatak Arc Specialist - Nunatak is without a doubt the most well known brand name when it comes to backpacking quilts. They make some of the better sleeping bag quilts available and use excellent materials, occasionally even offering 900+ fill down for their products. The Arc Specialist is an excellent 32 Fahrenheit quilt but if you're looking for a quilt consider just about anything from their product line.
Katabatic Sawatch - Katabatic is less well known than some of the more established brands on this page, but with some good reviews, quality materials and clearly well thought out designs you should seriously consider them when looking at quilts. The Sawatch is a 15 Fahrenheit sleeping bag and a serious contender in the quilts category.
Jacks R Better Stealth - Called the Swiss Army Knife of sleeping bags by Backpacker magazine, the Stealth is an excellent piece of gear. It's rated down to 40 Fahrenheit and is available in a 900+ down fill option, which will help save even more weight on an already very lightweight backpacking quilt.