Celebi

One of the most elusive Pokémon in history is finally available via wireless connection to download onto any fourth generation game! Celebi, the mystical time-traveling Pokémon, has come to join parties at GameStop stores all across America! And this event Pokémon has not one, but two features! First, by taking it to the shrine in Ilex Forest in HeartGold and SoulSilver, you'll trigger an event featuring a back story of Team Rocket and your rival. While not exactly stated in the English version of the games, the red-haired young man is actually Giovanni's son, as hinted in FireRed and LeafGreen. The event ultimately leads you to a battle with Giovanni himself, forcing you to start with Celebi, since it must be your lead Pokémon to trigger the event. Secondly, once you're through with Team Rocket for good, you can transfer Celebi to the upcoming games Black and White via the Poké Transfer and get a Zorua in Castelia City. Since Celebi starts at Level 50, you might not wanna use it in your party after you transfer it until you get enough badges, but you already knew that drill, did you?

...I still want Pokémon Black...