Due to my limitations in my bag (32 ltrs) I would like to know which one is both the lightest and warmest solution I can pack with me (for the torso area). I do not care about doing sports in it. I just need it after a hike or walk i.e. stationery positions only.
Down is the lightest and most compressible delivering the best warmth.
Down is the undercoating of water fowl. It provides the greatest amount of dead air space of any insulation material for its weight. A down pod consists of light, fluffy filaments that grow from one quill or point. These filaments intertwine and mesh, forming air pockets which trap air. One ounce of goose down contains 23,000 pods and two million filaments.
It comes in different Fills ranging from 450 to 900, also be mindful that manufacturers may also only put 70% down and 30% other insulation...The best are usually 90/10.
They also come with different weights for example the Patagonia Nano Puff is probably one of the lightest down jackets, then you get into something like a Mountain Hardwear Nilas, and then last but not least an expedition weight parka like the North Face Himalayan Parka.
Here is a Buying Guide that may help you with your decision.
Although I agree with AM_Hawk as down being both light and compressible, I'd also suggest layering your clothing. Trapping heat between layers keeps warm air near your body, even without using a specific material.
The first layer is packed tight next to your body and regulates moisture / sweat: It keeps you dry. For example, a tight t-shirt. Next, a layer to keep the heat next to your body. Down is a great material for this. Finally, a layer to stop the wind. This also prevents the layer underneath from cooling down. A windproof jacket, even without down insulation, should do the trick.
Although this sounds like a lot of bulk to carry, I've found it to be quite effective. Because of their combined effect, the individual layers don't have to be that heavy.