My most often worn Arc'teryx
Not for it's performance either, though that is impressive. It's because for cold Alaska mornings, this thing is supersoft lounge wear. Outdoors, it performs like a champ, with incredible warmth for something that compresses to the size of a rolled up magazine. For ultralighters, this is an excellent temperature range extender. Paired with a windstopper top/shell and and baselayer, this should keep you comfortable when temps dip in the 40s.