what is the lightest weight most comfortable sleeping bag on the market?

Answer 1

The bigger question to ask is what are you going to use it for. The insulation value is most important. A sleeping bag that keeps you warm at -40oF would be too hot to use during the summer; likewise a summer weight bag isn't much good in the winter. The more insulation; the more weight.

Answer 2

mountain hardware has the lightest bags out there.