If it's straight line quilting you are after ... I would use straight diagonal lines. Use a walking foot to make it easier.
The easiest way would be to use the corners of each rail as a diagonal, you may not even have to mark it. Start in one corner and go clear to the other corner. Then do the same for the opposite corner making a big X in the quilt. Then jump down to the next line of rails and do a straight diagonal line from end to end ... and so on .. working your way from the middle outward. You will then have a bunch of X's in each of the set of rails. I think that would look pretty, it's easy, fairly quick, and I suspect it should be within the recommended quilting distance of your batting (but check first).
Personally I would use a light weight neutral thread. Let the design in the quilt be the star.