Well what is "best"? How about someone practicing piano 12 hours a day for 70 years? The nature of your question is flawed if it implies there is a single best way for everyone. Everyone has their own goals and their own brain and hands to contend with and also life situations.
Your question also implies there is an ending to one's piano studies to "learn the piano in the shortest time possible". Unless ones concept as to what constitutes learning the piano is encased within a limited set of skills learned at a specific standard (which is quite a narrow perspective), there is no end.
There are axioms which tend to be consistent with the best way to learn, such as guidance from someone more experienced, consistent effort, setting goals, maintaining motivation etc etc, but exactly how these work certainly comes with variation, thus a universal "best way" also becomes various. It is not only various but also can change over time with individuals, so there is no fixed "best" way too and it certainly is fluid and changing.