I can honestly say I really can't decide.... There's things I love so much in every game. Emerald's epic region and Battle Frontier, Platinum's awesome story and improved gameplay and HeartGold's nostalgia meter off the charts along with awesome gameplay updates. And then there's Black giving the franchise a fresh update and completely obliterating anyones expectations of a Pokémon game.
I would probably have to say Platinum for its near perfection of the Pokémon formula. It fixed the great Diamond and Pearl's obvious problems and created an incredibly solid RPG.