80/20 cotton/poly blend batting is what I use. One thing to remember these days - doctors tell parents not to put blankets on infants, as an infant can wiggle enough to get under the blanket and suffocate, especially if they wiggle into the corner of a crib. So, if young parents today follow DRs orders, babies will not use their quilts while unsupervised.
I personally dislike 100% polyester batting. Some of it feels like brillo to me, and I imagine it scraping away the quilt fabric from the inside. But that is my wild imagination, as I've never heard any one complain about that.