I like the blue water soluble pencil for marking lighter fabrics and the chalk that comes in a mechanical pencil (Bohn) for dark fabrics. I've never had the blue not wash out even with a light spritz of water and blot with a wash cloth. You do have to be careful you don't iron it while it is visible however as it will heat set the blue marks. I've never done that but heard it from others.
I have used Fixon pens as well with success. You have to be careful of the red on some fabrics as it will leave like a faint bleach line. I've never had problems with them washing out or reappearing.
Agree that you should always test your marking method with the fabric you're using to make sure it will come out.