What's the lightest batting?
 

I'd like to make a super lightweight quilt - is there batting that's best for that sort of thing? I wonder if bamboo would be cooler than cotton or wool? Ideas?

How about just flannel sheeting...or some people make summer quilts and don't use a batting at all.

Which is just two layers - the top and backing.

Mountain Mist has an ultra-thin polyester batting I have used that is very nice. Found it very reasonably priced at JoAnn's. I think the loft is 1/4". Used this in a baby quilt and it came out very soft and light.

I bought some Fusi-boo on sale at Joanns and it was very thin, in fact too thin. I did like the fusible quality though -- it was very fast to sandwich (I was doing a QAYG) & quilt. I had a cotton pieced front and back, and used the Fusiboo for batting. The quilt turned out fine except it is much thinner than I expected and certainly not as warm as cotton. So I can say for sure that Fusiboo will be very thin and lightweight if that's what you're looking for.

Quilter's Dream makes a very, very thin batting which is available in different fibers (cotton and otherwise, I believe), and I have also used Theromore, which is synthetic, very thin and a very even thickness all over.

Pricey, but you could also look at silk batting. I'd think you would get the same heat retention as with cotton or wool without the weight. I've not personally used it but it was wonderful to fondle. It's on my 'someday' list for that most special quilt.

My experience as well. That's all I use on baby quilts now.

Hobbs Thermore. One time I split apart a Fairfield batting for the hood in a silk cape. I can't remember (now) which it was. But a poly cotton batting I will remember when I am not thinking about it.
I would definitely use a batting and not flannel as it would be lighter in weight and more drapable.

I think Cream white or Cream Rose is one of the lightest battings I have ever used. It is a Mountain Mist product and 100 percent cotton.