What's the best material to use to make cushion covers?

Look at curtain fabrics. They are usually more durable than dressmaking fabrics. You could get a plain material and decorate with fabric paint, beads, embroidery.

There are some fabrics you can buy with square designs specifically for cushion covers but the designs are generally very twee and are too small for my taste. I like LARGE cushions.

Cotton is the easiest to clean and more durable for everyday. If you are looking for something fancy, than maybe a faux remnant/fabric of sorts.

It all depends on the usage, the style, the room type. For example, you wouldn't use canvas for cusion covers on a victorian style couch, and you wouldn't use velvet to cover boat cushions.

I'd go to a fabric store and take a look. They have some quite nice things sometimes and don't forget to use something sturdy so they don't fall apart 2 days after you make them...:)

curtain or light weight upholstery material .i buy it at yard sales & flea markets & then make pillows or tote bags with it & they look good.

Velvet or crushed velvet would be nice.