Look at Sierra Designs down bags but ONLY the ones with a down DWR (Durable Water Repellant) treatment. Down is the best and most comfortable insulator and the Dri-Down or Down Tech treated down dries FAR faster than regular down and the Sierra Designs bag as tested by Backpacker Magazine dried faster than the synthetic bags!
Down keeps its loft for many more years than the best synthetic (i.e. Climashield).
TIPS:
1. Never, ever roll your bag. Instead stuff it in the stuff sack foot first for transport only. Take it out of the stuff sack as soon as possible and put it back as late as possible. Stuffing ain't great for loft but it has to be done.
2. Store it loosely in its big storage bag, hang it or lay it under your bed.
3. Wear only "sleeping clothes" in your bag to keep it from stinking.
4. Spray the inside of your bag with FABREEZE Anti-Microbial freshener and air it out well at home after using. This stuff works well on the insides of Gore-Tex parkas and pants and boots too. After a few days there is no scent left.
5. To make the outer shell of your bag very water resistant spray it with REVIVEX. It's what Gore specifies for their garments and it's the best I've found.
6. If tenting get a Thermarest Trail Pro mattress for enough warmth beneath you. If bitter cold (below zero) you may have to add a Thermarest Ridgerest closed cell foam mattress under it.
L.L. Bean also sells down jackets and (I think) down bags with one of the DWR treatments listed above. Try to buy all your down gear, jackets, pants, etc., with DWR down treatment.
Western Mountaineering and Feathered Friends sell THE highest quality down bags made - if you want to pay for it. I don't think they have any DWR treated down. I have a W.M. Megalite 3 season bag and love it for backpacking. W.M. will "overstuff" a bag for a small fee if you ask when ordering it directly from them. That will give about another 10 F. of warmth. They did it to my bag and it came back looking like the Michelin Man.