Way too open question, probably, as it depend on you, how much you are willing to pay, what you like, and so on.
I personally am fully into slight overkill on the bag for a given situation. It you already have one that is slight overkill, then that is the most "economical" sleep solution in terms of weight and everything else. But I'd personally suggest a merino wool hoody. Synthetic is ok too. Wear that over your innermost layer, and use both hoods (hoody and outer jacket). It will have the advantage that it will be a great layer for hiking as well. Full or half zip. Weight going up into expedition weight if you need that much. But you will have much more flexibility with stuff you can also wear while hiking. Vests are great for sleeping and hiking too, and provide a layer right where you need it most. In the end you will have to try things.
Since I live in the area, I can tell you what I would take if I went out tomorrow night.
I'd be ready to sleep in all of it, but hope it would usually not be necessary to do so.
Patagonia sikweight merino crew
Silkweight or better long underwear bottoms
Ibex indie merino hoodie
Montbell UL down parka
WM Hilite (35 d.) Bag. Or maybe I'd just take a beefier bag since it would be only a few oz more. You can always take layers off, but once you are out of insulation....
This would probably be good down to 25 deg. winter in high mojave > 2000 ft elevation, or coastal range with super high humidity and a cold snap like last week.