What is the best wash to get old water-proofing stain off a deck?

TC us right, stain works into the wood. Some detergents with a pressure washer can get some out, but not all.  Sanding can also get down enough to remove the stained fibers, but it is a lot of work. The stains that also cover are going to be your best bet after getting all the loose crap off and such.

you can rent a pressure washer at your local home improvement store or else the local equipment rental place.  and, frankly, nothing less is going to accomplish much.  -- grampa

There really is no good way. 

A stain is meant to penetrate wood. To effectively  remove it you would need to sandblast which would destroy the surface of the wood ....then belt sand it to get a decent finish.  

You could sand it, but that would take a ton of work. Pressure washing may work, but its unlikely you would be able to remove all the stain ...again lots of work.Not knowing the deck or the shape you may consider flipping the boards.... most likely it is not stained underneath.Or just get a solid color stain and go over the deck with a color you like.