Which Pokemon game is the most diverse and challenging? I'm talking about content, the variation of Pokemon available all around as well as the battles and gym leaders.

I was considering up picking a Pokemon game.
The last one I played was Emerald, which was neat as well, but that was probably 10 or so year back now.
I loved the post game content in Emerald, but it's not mandatory really...

Any recomendations, and if possible could you back up you answers with some reasons?