You can catch Arceus in Pokemon Platinum Version, but only by obtaining it through a Nintendo event, unless a GameShark or Action Replay is involved. Legitimately, the event gives you an Azure Flute when you talk to the green person next to a Pokemart in any town, and when the flute is used at Spear Pillar you can fight and catch Arceus.
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You can catch Arceus in Diamond, Pearl, or Platinum.
The only way to catch Arceus in Pokemon Platinum is by using the Azure Flute or cheating. The Azure Flute is an event item that sadly was never distributed. This means the only way to catch Arceus is by cheating.
Arceus is not an available Pokemon in Pokemon Sapphire. Arcues is only available to Pokemon Diamond, Pearl, and Platinum via an event.
The strongest Pokemon in Platinum is probably Arceus, to get Arceus, you need a mystery event, but that event is finished now. :( The second strongest Pokemon would probably have to be Giratina, or probably dialga or palkia.
You can only get Arceus by an Event.