Unova for me. It wasn't just the story of N, the illegal Hydregion of Ghetsis, or the fact the Gym Leaders were more than just people you had to fight for Gym Badges. The rivals Cheren and Bianca were there to challenge you and support you(much better than those in Kalos) and the music in Unova is dynamic(with the sequels really experimenting with Gym Leader themes: Elesa's theme anyone? Final Gym Leader Pokémon too btw). The cherry on top for Unova is the fact they added a ton of new Pokémon to the game just for the new region. Some were rather questionable design-wise but no other region, including KANTO, added more Pokémon to the game than Unova did.