What is the best sleeping bag for the Appalachian Trail?

The best sleeping bag for hiking the Appalachian Trail should definitely be a Down sleeping bag (rather than synthetic) to reduce pack size and weight, and also to be rated for 20⁰ F (-6.7⁰ C) degrees to handle most weather conditions (cold-weather/all-weather).  Here are some of the most popular sleeping bags for long-distance hiking of the AT:

• Western Mountaineering UltraLite Sleeping Bag: 20 Degree Down, 850+ fill down, (regular) 1 lb 13 oz
• Feathered Friends Nano 20 Sleeping Bag: 20 Degree Down, 900+ Goose Down, (regular) 1 lb 12 oz
• Marmot Helium Down Sleeping Bag: 15 Degree Down, 800 fill down, (regular) 2 lbs 1 oz
• Kelty Cosmic 20 Degree Sleeping Bag: 20 Degree Down, 600 Fill Power DriDown, 2 lb 9 oz

As for sleeping pads used on the Appalachian Trail, the most common pads are made by Thermarest. The foam pad most popular is the Therm-a-Rest Z Lite Sol Mattress. The most popular inflatable pad is the Thermarest NeoAir X-Lite.