I use old-fashioned Mountain Mist Blue Ribbon 100% cotton batting in my quilts. It is not thick, but it becomes softer with every washing. It does need to be quilted fairly close together and it is a flatter batting than some people like. It gives the soft, crinkled antique effect you see in old quilts.
I made a baby quilt with this over 20 years ago. 15 years later I happened to see it again, after many trips through the washer and dryer. The binding was frayed in places and the fabrics had faded, but it was so soft it felt like hugging a cloud!
Hobbs Heirloom 80/20 is a nice batting if you want a fluffier effect. In general, I don't like to use a batting thicker than this one as they become harder for me to quilt (both hand and machine). I tend also to stay away from battings that are needle-punched through scrim, such as Warm 'n' Natural, because the drape is stiffer.