Jump back 10 years and I'd have said 'Down, Down, Down' but..... I've grown a social conscience and technology has moved on..... synthetics can offer the same insulating properties and yes 'up-cycling' / recycling has found a place in this space so.... what do I buy and recommend:
The mid-layer is the insulating layer, it is about capturing your bodies natural warmth. The easy part here is that I don't buy 'winter' grade, in the UK we rarely need it, I'd rather have two 400-600 fill jackets that I can alternate or double up on the rare occasion that it's that cold here or the more common occasion that I'm in the snow.
Also worth bearing in mind is that down jackets are great for trapping warmth but not so great, vs fleece, for blocking wind. For me that actually means I keep two mid-weight (packable) downs and a lightweight fleece to combine for various conditions:
Final advice: chest pockets are useful and skip the hoods; buy a hat and neck warmer of some description. Hoods catch rain and snow when not in use and get in the way when you need to hear your mates, the wildlife and the silence of the nature around you!
For me it's any two of :Fjallraven Expedition Pack Down, Rab Microlight Primaloft for Down and Ternua Yukkon for recycled Down and a Berghaus for fleece for wind protection if an outer layer is not being taken: all available in Ladies and Gents fittings and a variety of sizes!