Sinnoh
Sinnoh is the region in which Pokémon Diamond and Pearl are set, and just recently, Pokémon Platinum. It is a widely varied area, with a large chain of mountains dividing the region in half. The most notable of these mountains is Mt. Coronet, the peak of which is the legendary Spear Pillar. Either Palkia or Dialga will apear eventually, depending on what game you have. If you have an Action Replay, you can also get Arceus by using the Azure Flute to allow entrance to the Hall of Origin.
In Super Smash Bros. Brawl
The Pokémon stage, Spear Pillar, is located in Sinnoh. Spear Pillar is supposedly the area where the Sinnoh region (if not the entire Pokemon Universe) was created and is thus very important in the game. The protagonist of Diamond or Pearl battles the leader of the evil Team Galactic, Cyrus, here and captures or defeats either Dialga or Palkia. Much later in the game, it is possible through the use a special item to battle the strongest and rarest Pokémon of all, Arceus. Arceus does not appear in Brawl, as it hadn't been officialy announced, though Dialga and Palkia both appear as background characters on Spear Pillar. Cresselia is also seen appearing and using a move similar to a Razor Leaf made out of Lunar Plates.
The Sinnoh region may also be home to the stage Pokémon Stadium 2; however, this stage could be practically anywhere, considering that Pokémon battling is common to most of the Pokémon world.
