Depending on the quilt, I use Perle Cotton, 1/8" or 1/16" ribbon, embroidery floss, whatever I decide goes with the 'feel' of the quilt. When I do most of baby/child quilts, I back them with flannel-back satin and use the ribbons. Makea a beautiful quilt and the ribbon blends in to the satin backing.
I always tie with a surgeon's knot - the square knots have come apart on me on quilts that are used and washed regularly. HTH