If you bring over the special Arceus given away for the 12th movie, you will unlock Dialga, Palkia or Giratina. This is done by going to the Shinto Ruins, hidden somewhere in Johto with Arceus on your team. You will encounter Cynthia who will tell you that Arceus has the ability of creating certain Pokémon. These Pokémon are Dialga, Palkia & Giratina. You have a choice of the Pokémon you can create but you can only create one so choose wisely.
They are all at Level 1 and holding their respective orbs, so it is a truely personal preference based decision and they all have moves they don't normally know.

Dialga Lv. 1
Special Attack: Metal Burst

Palkia Lv. 1
Special Attack: Hydro Pump

Giratina Lv. 1
Special Attack: Shadow Sneak