Angie, I sometimes take my quilt tops to a longarm quilter, and normally ask her to decide what wadding she wants to work with.
If I have to make a choice, I consider what I want my quilt to look like, AND what properties it should have. For example, I picked a low loft cotton batting for a bed quilt that went to Southern California, but a higher loft wool batting for a quilt that now lives in Alaska. If the loft is very low, the quilting lines will not stand out so much. Very high loft batting is really only suitable for quilts that get tied rather than machine or hand quilted.
Choosing your batting is a very personal thing, and you should get lots of different answers here.
My personal favourite (if there isn't a strong argument for something else) is Hobbs Heirloom 80/20. It's a predominently cotton batting with some polyester. It machine quilts very nicely, and then washes well when the quilt is in use. I also find that the longarm quilters I know like that particular batting.
Good luck with your choice!
From the edge of Sherwood Forest, home of Robin Hood
I am new to quilting and have just finished the top of a lap quilt. I am going to have somebody machine quilt it for me, and I was told that I should bring the batting. Do you have any recommendations on what to buy? Thanks so much! Angie