For gameplay, either Gen 2 or 6, the former in terms of post-game material whilst the latter in terms of overall game mechanics (though if including remakes, you could logically replace 2 with 4, especially since the DS doesn't have the limited lifespan of a Gameboy cartridge). Plot-wise, I find Gen 5 is pretty underrated, since along with removing a few constantly recycled tropes of the franchise with it's initial Pokémon League challenge it also had time to be more thought provoking in probing on the moral issue of Pokémon battles.