What is the easiest way to tear out a bathroom subfloor?
As HerrBag suggested, it's much easier to go through the ceiling below than to open up the floor. Cut open the ceiling back to a joist on each side, do your plumbing repairs there, and install some replacement drywall. You'll have to mud the joints, which is a bit of an art to make them disappear. And then prime/paint your patch.

Trying to tear out the floor from above would require removing everything that's on the floor, it's much more costly to replace the flooring, and structurally you don't want a lot of joints in your subfloor.