I tried Crayola--but the green didn't wash out.
I love the soap slivers because they are free and easy to wash out--if I'm washing the quilt. Not good for gifts.
I really love the air erasable pink Trace'n Mark by Nancy Zieman. Humidity level determines how long the marks remain.
And in most situations I like Frixion pens, but I found out the hard way I need to test the fabric as sometimes a white line remains after ironing. Great for white fabrics.
My favorite quilt marker is Dritz Tailor's chalk in the green and white pen. Very thin line of chalk that will erase but stay while I quilt. However, I can't mark the whole quilt as it does rub off.
I'm not sure there is a really perfect marker, but there is the perfect marker for every situation.