Go by price. The heaviest, best quality will be the most expensive. It shrinks a lot so I suggest pre-washing with hot and buying generously.

I prefer a flannel called diaper flannel to back a quilt. Its a heavy, double sided flannel. I used to be able to get it at Walmart, but, now I have to look for it. I used to pay about $3.50 a yard. I'm sure to prewash it. It only comes in off-white and white. I use it a lot as a backing when I make flannel crazy quilts.