I always remember my first quilting teacher telling the class, "don't just look in craft fabric", check out dress fabrics and even curtain fabrics. I've used some of these other fabrics as well as muslin.
Now I know a lot of people will be appalled to hear this but I love using muslin. It's strong, 100% cotton and it gets softer and whiter the more it is washed. I usually wash it 6-8 times and leave it to soak over night in the washing machine with 200 ml of Demestos (hospital grade bleach). LQS sell quilters muslin for $25 - $30 per metre. This is just muslin that has been processed further.
Mary Ellen Hopkins told us " you can use any fabric as long as you are willing to prepare it". She described her favourite at the time as a wibbly wobbly slush coloured fabric. I made a summer quilt for my DGS when he was in day care. The front was a piece of curtain material with teddies on tartan, white flannelette for batting and the backing was a maroon with tiny spots that I brought to the front and made a 2.5 cm wide frame. It was adorable.
At the end of the day, unless you are making it to enter a competition, it is your quilt and you can use whatever you wish. Don't be dictated to by the quilt police. Just enjoy your quilting making.