A high loft down bag, mummy style, has been the gold standard for a warm backpackable sleeping bag for many years. There are synthetic fill bags that come very close, basically being only slightly bulkier for a given temp rating. A lot depends upon how critical the weight-warmth-bulk relationship will be in your activities.

Down bags are quite durable with only modest care. I often use a light bag liner or a set of long johns to keep the bag clean. I rarely,if ever, need to wash a bag.

My coldest trip ever was a mountaineering patrol of Denali in 1987. I used a synthetic bag, rated to -30 (courtesy of the NPS). -20 was routine, and one stormy night got down to -80 with wind chill. Bulk was not critical, because we were pulling sleds. Still retain all fingers and toes.