Probably Arceus. He has the highest Base Stat Total of any pokemon, he can be any type, and he has a giant movepool. But he can only be gotten through events (or hacking)
Other than that, maybe Kyogre. His rain boosted water spout can do extreme amounts of damage, killing basically everything that doesn't resist it (except Blissy)
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The strongest Pokemon in Pokemon Platinum depends on your style. First, if you like defense, you should choose either Regice, Regirock or Registeel. If you like attack, you would go with deoxys attack form. If you like speed, you should go with either ninjask or deoxys speed form. If you like health, then you should definetly go with blissy or snorlax. Hope this helps!
The Frontier Brains and the Champions.
Raiku the electric dog on leaf green and fire red versions
The rival the third time you battle him in survival area
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.
mew is the strongest Pokemon