The three most important features of a sleeping bag should be warmth, weight, and packing size. The Therm-a-Rest Hyperion ($340) gets an A+ in all three categories. It's filled with RDS-certified hydrophobic 900 down, weighs around a pound, and it cinches down to the size of a water bottle.
The Hyperion has a Comfort EN rating of 41 degrees fahrenheit, meaning that's the lowest air temperature at which the average woman can sleep comfortably. So technically, it's a 3-season bag, although with a good tent and pad, you could use it year-round in most climes. Also important, I found that if I became a little overheated at night, the hydrophobic down dried out quickly in the morning.