Cotton, with either cotton or flannel as a backing. Flannels make great rag quilts. My daughter just made one for a new baby and it came out beautiful!!
I like cottons for the front of the quilt and flannel for the back of the quilt. Personally, I think the Minky is hard to quilt with, but it does make a cozy backing.
I use whatever strikes my fancy when I am selecting fabric for one. I rarely use a pattern for these so I first pick the fabric and then the desighn. It is yours to make so use what you want.