
NEMO
Disco 15 Series
Spacious spoon shape, but bulky and inconsistent warmth.

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Just got back from Algonquin where the temps dropped to 3 degrees C. I was comfy in my fairly new -9 degree C Nemo down bag wearing long base layers only, but I felt if it got below freezing I would need some additional layers. Other commenters are giving good advice when they say a good sleeping pad makes a big difference, especially if the ground is already cold (not just air temps dropping at night). Don't trust temp ratings of cheap bags and aim for something with a rating 15 degrees below the coldest you could expect it to possibly get.
Nemo down 15F is my favorite. I felt warm sleeping when outside was below 30F. Wait for the next sale in REI you can snag it for nice discount.
I love my down 15* Nemo bag. It's a huge warmth difference compared to my synthetic 25* Nemo.
I’ve used an EE 30° quilt below freezing with a Tensor Extreme and been fine. I’ve used a 15° Nemo Bag with a tensor elite above freezing and been cold. I am a believer in the sleeping pad makes the biggest difference solely based on my own personal experience.
I can vouch for Nemo Equipment bags (I have 3 down ones and one synthetic for different temperature ranges). I can also vouch for Marmot down coats (for in-town use not for winter backpacking or camping) but I have never used a Marmot sleeping bag. My 3-season backpacking bag is a Nemo down 15F version that is probably 10 years old now and it is great except for in extreme heat and extreme cold at night. I have an old Eddie Bauer Flying Squirrel Sleeping Bag (modified with velcro) that I use for extreme heat and other Nemo down sleeping bags for winter backpacking. Nemo has a lifetime warranty on their equipment.
I like the Nemo sleeping bags that have the gills. You can open them when it's warmer and zip them up as the temperature drops. Works much better for me than just opening the side zipper, which I find let's too much heat out.

NEMO
Disco 15 Series
Spacious spoon shape, but bulky and inconsistent warmth.
Hammock Gear
Classic Burrow 20°F
Budget-friendly quilt; very warm, but not ultralight.

NEMO
Disco 30
Roomy spoon shape, but bulky and prone to zipper snags.

Katabatic Gear
Alsek 22°F Quilt
Premium quality, very warm, but high price for weight.

Neve Gear
Waratah
Great warmth-to-weight and value, but untreated down.

Ranked #1
NEMO - Disco 15 Series

Ranked #1
Kelty - Tru.Comfort Doublewide 20
Ranked #1
Enlightened Equipment - Revelation APEX

Ranked #1
NEMO - Disco 15 Series
Ranked #1
Hammock Gear - Classic Burrow 20°F

Ranked #1
NEMO - Disco 15 Series