For synthetic the Wiggys Ultima Thule is a -20 and lands at 5 lbs. Down bags such as the Western Mountaineering Kodiak and Antelope MF ,NorthFace Inferno will be lighter by almost half. Only thing with down is if it gets wet it looses it insulative properties and will take a year and a day to dry. Some guys take a bivy bag to go over it for protection but that kind of negates the light weight theory. Synthetic however if hung in the sun will have you back in the bag quickly. I have both, my down bag sleeps nicer but on Alaska or extremely remote trips in Canada the Wiggys gets the nod. If I will be in walking distance to the truck I take the down. Wiggys does offer lighter weight options as well for synthetic and western mountaineering has a down bag at 2.06 lbs called the “Versitile 10”.
I have the chillkoot 0 degree bag. I’ve had it down to the high teens and still been warm in the bag. Extremely happy with it after about 15ish nights in it.
I have the Kodiak and versalite with overfill for light, kifaru slick and wiggys super light for not so light plus an alps mountaineering -20 for heavy. All work great situationally.
I have had the versalite 4ever...wm refurbished it 4free a few seasons ago and added some down. Loft is still fantastic but I take care of my gear. Correct washing is important.
I recently purchased a Stone Glacier 0 degree bag. About to start using it so cannot speak to its effectiveness for myself however build quality and the reviews across multiple outlets are excellent....