Undoubtedly HeartGold, it finally let me finish the Johto games after my internal battery ran dry on my 7th badge when I had Gold. The original G/S were brilliant games and all the added features in HG/SS just improved upon that hugely (I really enjoyed the Pokéathlon). Plus I got to use Typhlosion in the main game which is always cool.
Diamond & Platinum were great games, but they're lacking in comparison unfortunately. I spent more time on the two than any other game though and Sinnoh was undoubtedly the best region, it's just the games aren't as good as a whole.