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Disregarding the supposed doomsday nonsense and just doing this for fun:
>if legendaries are allowed
Dialga/Mew: Dialga can become immortal and invincible by exploring every possible era's technology, and Mew can transform into Dialga without having even seen it if it truly contains the DNA of all Pokémon.
>if legendaries are banned
Ditto: As broken as Dialga and Mew if we go by the strongest Pokédex entries (free transformation) and still extremely tough by being able to turn into things like rocks if we go by the weakest entries (failure rate if going by memory and/or having to mimic something that's there at the time).
>if legendaries and Ditto/Smeargle are banned
Charizard: Capable of flight (always a dream of mine), holding objects with its hands (would make me feel a bit more human and is generally useful), cooking, sterilising water, cauterising wounds, and other applications of heat of which I am not aware. If I end up getting captured, I might also be able to go Mega someday for some extra power and perhaps have a choice as to which form to take.
Honorable mention to Rotom for its object-reliant flexibility of functionality and general ghost powers.
