Victini.

It's Diglett.

Lore-wise it's Arceus.

Arceus

Also Mew doesn't "evolve from all Pokemon" it has the DNA of them

Lore-wise, it's Arceus.

Creation wise, it's Rhydon

Mewtwo is the first pokémon in the anime intro and Charizard in the japanese intro

Well I mean by technicality, Rowlet hypothetically would not make the list here, right? Yes, it IS number 1 in the Hisui dex, however, As we can see by the very OPENING of the game, Arceus existed before Rowlet.

This logic could ALSO apply to Bulbasaur as well, seeming as it's essentially just the same rodeo. However Arceus was not IN gen 1 {as it was introduced in gen 4} so I'm not entirely sure... But since Arceus was in Hisui, possibly a few decades ago from Modern Kanto, I can assume Arceus existed before Bulbasaur.

Remember people, first in the pokedex does NOT equal first pokemon.

Anyway, Lore wise, it's Arceus. Actual design wise it's Rhydon.

Wait Rowlet was a first in something? I think a better fit would be the first pokemon to not being able to evolve because of an everstone he ate