I'm not sure there is a "best", but there certainly are some "worst". I've used a variety of things for backs. Old sheets, cheap muslin (that's a worst!- the cheap 3.99/yd at Joanns), LQS fabric, kona muslin, flannel, and my overall favorite, Legacy Premium Muslin and yes, its from Joanns. I did buy some beautiful wide backing with butterflies this summer. Its a brushed cotton and I love it, but would be terribly warm for warmer months. For backs which are most frequently near your body, I'd stick with all cotton, use what you like and just stay away from the really cheap stuff, it won't hold up in the long run.