I use a variety of different middle layers for my quilted garments, depending on the seasons I want to wear them and how much definition I want in my quilting.  For warmer weather or for wear with sweaters, I like to use either muslin or well-washed flannel.  This gives a lighter-weight garment, but doesn't allow much definition in the quilting.  For more definition or for colder weather, I like a thin cotton batting with a lot of drape.  Quilter's Dream Cotton Request is my favorite, but I have used many others to good result.  I usually use a lighter layer in the sleeves, like muslin or flannel.