Don't forget about shopping at thrift stores. My Aunt used to buy wool army blankets and use them for batting.
I have used flannel many times. Today I'm finishing up a quilt that has a pieced top, flannel, then minkee backing.
I have also used fleece for batting. Grew up in a very hot climate and my grandmother used sheets for batting. She also used sheets for backing too.
I have also made quilts without batting. On those they either had fleece or minkee backings. I don't like a thick batting when I use a heavy backing.