I have made five quilts in the past year and have used Legacy Brand's bleached wide muslin. It is the same base fabric that they use for their quilting fabrics and has a great hand. Nice tight weave, opaque, and a really good price, especially with a coupon at Joanns.
BTW, the quilt in my avatar is one of the quilts using it. I posted a picture of my son's quilt too. There it was used with two batiks so I want a pure, solid white and it definitely fit the bill. HTH