Wool is wonderful, especially if you're using it in cold and wet weather. I'd also do heavy flannel for the backing or even a nice fleece, if warmth is critical. Minimal quilting (just slightly over what the manufacturer calls for) so that you get the maximum insulation. I've also used old blankets -- my warmest is one that has an old Army wool blanket in the center (from WWII), and 2 layers of cotton, as it was recovered over the original cotton. Heavy, heavy, but WARM! I love that blanket, as I didn't do any piecing, just used decorator fabric to cover over the old fabric, and stitched to make sure it all held together.