I like my Marmot Helium a lot; it's kind of my go-to bag unless it gets really cold or wet. Nothing wrong with the better REI bags either, and they sure stand by their guarantee. I have a Kilo Plus, but I think REI stopped carrying it. The Marmot and REI's regularly come up on sale. The other premium bags, while terrific, don't. On the other hand, everybody I know with a spiral bag gets pretty misty over it.
Don't forget one or even two good insulated pads. When it's really cold, I will carry both a Z Lite and a light insulated blow-up pad. Makes a huge difference, and you can sit on and beat up the Z Lite in a way the insulated inflatable couldn't take.
And don't ever, ever forget a warm hat or balaclava. I have, numerous times, and paid the long, dark night's price!