Arc'Teryx Atom LT Hoody.
I looked at the Patagonia Nano Puff, Rab Xenon, and MontBell Thermawrap too.
–The Nano Puff is a classic, but fit is a bit boxy and is a little heavier than the Arc'Teryx piece.
–The Rab Xenon is a solid offering, but is sized very much as a belay jacket, which didn't work for me. If light is right, this is your pick though.
–MontBell is reportedly less warm
The Atom LT fit me the best, has an extra interior pocket, and has an outer fabric that I really trust while still being light. It's also as light or lighter compared to the above options.