Suppose you needed to pack an insulation layer in case it got colder and you need to add an layer. What would be the lightest? Quilted down? Fleece? Wool? Some high-tech synthetic?
I've been doing some backcountry stuff the last few weeks. Warm going up & then cold at the top while taking skins off etc. I throw a down puffy in the pack. It's light, stuffs in the pack just about anywhere, easy to get on & off & warms me up quickly while still breathing.
Nothing can touch high fill goose down (by weight) Something in the 600 to 800 fill range will give you much warmth with little weight or bulk. But keep it dry and be able to ventlate as required to keep from saturating the down.
Yup, best insulation-to-weight = high-loft goose down. But you're screwed if it gets wet. Needs to be laundered with special down wash for best longevity.
Slightly inferior insulation-to-weight = primaloft (synthetic fill). Still insulative if wet. But not durable under repeated compression/laundering cycles.
Another step down in insulation-to-weight = structured fleece. Still insulative if wet, very durable to repeated compression/laundering cycles. Available with wind-resistant faces/layers to function as a softshell. Polartec Wind Pro is my favorite.
Slight step down from the previous = wool, esp. cashmere. Feels great, somewhat thermo-regulating. Loses some insulative ability when wet (not sure about this one).
Lowest insulation-to-weight = conventional fleece. Not because its bad, just heavy.
Here's what my wife uses---a down parka under 5 ounces. Stuffs to the size of a beer can.