I would say that a down sleeping bag compresses most, as well as being lighter in weight than a synthetic (i.e. with a fill of man-made fibre) one of a similar temperature rating. You need to be aware that with down you mustn't let it get wet, or it will lose it's insulation properties, as well as take ages and ages to dry, whereas a synthetic will still keep you warm and will dry fast.
However, I haven't kept an eye on the current market, and for all I know there might have been some super technological advancements made to synthetic bags in terms of bulk and temperature rating, so I am just as keen as you to see what other forum members suggest.