Making its third appearance on my list (a perfect record!) is the MontBell Super Stretch Down Hugger #1 (link goes to current version).
What I wrote three years ago is still true today:

One of the biggest struggles I’ve had as a backpacker is learning to keep warm at night. I have a slow metabolism and bad circulation, so it didn’t take me long to realize that a sleeping bag rated to 20 degrees is only good to about 35 for me. I’ve also learned other tricks, such as stuffing the dead space with extra clothes so I don’t have to warm up all that dead air. The MontBell UL SS Down Hugger bag has been a godsend in the sleeping bag department. With the stretchy design the bag automatically shuts down a lot of that dead space so I warm up much faster than I do in any other bag I’ve owned. They also seem to rate their bags to a comfort rating rather than a survival rating. Mine is rated to 15 degrees and I’ve had it comfortably into the low 20s – I wouldn’t even try that with my other 15 degree bag (which I’m not comfortable in below freezing). On top of that, it only weighs 2 lbs! Just don’t pay attention to the price (ouch…)

I’ve used this bag for five seasons now and it still lofts like new. There is no down feather leakage and the external material has remained free of snags. These bags are on the pricey end of the range, but this is a piece of gear I wouldn’t hesitate to recommend to anyone – the quality of this bag is worth the cash.