Easiest and Fastest??
Not sure where you are going with this one?
If it is purely for your own interest, the "easiest" and most straight forward (minimum fuss) that I have done, were the Canadian and the US. Very sensible approach to licensing. They are not easy, but they are relevant, study material is clear and unambiguous, actual test questions are used for study and taking the tests is very uncomplicated - you walk into a testing centre, pay a few hundred dollars and take the relevant test - kind of like it ought to be.
If you want difficult - go to Europe.
If you intend to continue in Oz, I would make the progression there, you are already in the system, familiar with the airspace and can presumably do the study and flight training/testing from home. That has to be easier and faster than going to another country then converting back to an Ozzy ATP.
tam