The batting is filling in the middle of your quilt sandwich and the type you choose will determine the thickness and warmth of your quilt. There is no wrong or right batting to use - its all about personal preference!
The most common quilt batting is made of polyester, but you can use cotton or wool too.
Wool - very warm and light. It also holds its shape really well and bounces back if crumpled. Wool is a natural fiber and resistant to creases. However, it is one of the more expensive batting types.
Cotton - very soft and comfortable
Polyester - holds its thickness and shape best when compared to other types of batting.