I stick with the classic - old school Tailor's chalk, the kind that comes in a square. I have it in white, blue, red & yellow and it always washes out with no problems. That stuff has been around for forever, I consider it to be tried-and-true.
I also like the blue water-erase pens, but I have to be careful not to forget and iron those marks in. So often I stick with the chalk just to be on the safe side.