Question

Is Crystal VC the only Pokémon game you can get all Legendaries and Mythicals in one game?

Comments

Could you elaborate?

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Not a full answer, but https://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_by_availability You can't get Mew in Crystal, so I think you probably can't get every mythical in any game.

Answer 1

Like sumwun said, you cannot get a Mew in Pokemon Crystal, which makes your question invalid.

However, in all of the Gen 1 games, you can catch all of the 4 legendary Pokemon, and a Mew through glitches, no hacking involved. In all of the Gen 1 games you can catch all of the Legendary and Mythical Pokemon.