Wool is the warmest. Silk is also known for its good warmth... but the amount of silk in a batting can vary and get pricey. If you want warmth with out the $$$ of wool or silk... go with poly. I double bat many of my quilts using a layer of warm and natural and a layer of poly.... very warm .. the quilting shows up great. An additional plus for poly is the dry faster in the dryer. A down side for double batting is if you are going to quilt it yourself on a regular machine... it too much to get under the arm of the machine. I double bat many of the quilts I send out to the Long Arm Pro.