I dislike these "what's the best ______ for every game" because people always fail to define what defines "best." Rarely do people take anything more than gym battles and the elite four + champion into consideration, and if they add anything more it's usually just rival battles or something. Should "best" take into account other trainers? Wild Pokemon? Surely best is also dependent on the other Pokemon on your team, right? Are we also taking into consideration how much time you might spend in each part of the game? What defines a part of the game? It would be easy to divide it into 9 parts for each of the gyms and the elite four, but are those parts equal?
Can we please define these factors first, before trying to answer this question, as well as any other questions?