Consider some of the three in one jackets from places like LL bean or Eddie Bauer. Separate fleece lining, waterproof outer jacket. You can wear the fleece, you can wear the jacket if you don't need the warmth, or you can wear both if it's cold and rainy. To keep the weight down, you can wear the fleece and not pack it.
I have one, and I love it. I'd give up something else in the suitcase to carry it.
The lightest for the warmth is down. and you can compact it very tight. Again. LL Bean type stores offers some great choices. If you have an outlet for any of them near you, some of the savings on the winter stuff right now is incredible, you may find some great deals on their down, and they may be the only places you can find cold weather clothing this time of year.
I stocked up on gloves and mittens last week, priced at $69, they were $5 a pair. They said the entire lot of them sells out in less then an hour every time they get them in.