Western Mountaineering, if price is at best a minor consideration. I've found all WM bags to be about 5 degrees conservative - that is, the WM Megalite is really about a 25 degree bag, which you can push down to 20 with long johns, stocking cap, and maybe a down vest - or, as I did last weekend, 15 wearing a down jacket and down pants over longjohns inside the vest. (The WM Flash hooded jacket and pants are great, when you get ready to look for insulating garments.)

If you decide to go with a 20 degree bag, try the WM Alpinlite. It has a draft collar that fits around both the front and the back of your neck, and cinches down separately (which means you have 4 cords, one each for the hood, chest, front draft collar, and back draft collar.) In this, I stayed warm with longjohns to 10, and to zero or just below with the down jacket, pants, and booties.)

The Marmot "Element" series would be my second choice (Hydrogen, Helium, Lithium, Atom); their ratings are pretty much dead on.