Warm & Plush 100% cotton batting. Cotton is almost always the heaviest batting and Warm & Plush is the thickest, heaviest of the cotton batts. It is sometimes a bit difficult to locate in local stores, but I've seen it sometimes at JAF and I buy mine by the roll from Walmart.com. I love heavy blankets & W&P is my absolute favorite!
My second choice would be a blanket from the Army/Navy Surplus... but then you're usually limited to darker fabrics -- at least for the blankets that they sell in the local Army/Navy stores by me.