The answer to that question is going to depend on who you ask.

My choice is flannel. I do not like synthetic fabrics at all. If you live in a place with little to no humidity (8% is considered high humidity here) then anything with polyester remains full of static electricity and you can see sparks shooting around as you move under the blanket at night. I find flannel to make a really warm quilt when I use it as backing and I don't have to worry about getting shocked when I turn over.