Hoenn by far, Emerald was my first game and Hoenn just has so much stuff to explore and such great variety between locations. I actually like Sinnoh, it had a lot of history and lore behind it and the Battle Zone (the big island) still stands as the greatest postgame of any Pokémon game (looking at you, X and Y, there's nothing to do after the credits roll.) Unova was also a lot of fun to explore, and N is the best antagonist of any Pokémon game yet.