Okay, better answer:
Rhydon: The first Pokémon designed, the most correct answer.
Bulbasaur: The first Pokémon, as in "its number is 001".
Mew: The first Pokémon, if, for some reason, you believe that a floating pink cat embryo came before a literal cell in Darwinian evolution, which leads us to
Solosis: The logical first Pokémon from a realistic perspective, being a cell and whatnot.
Arceus: The first Pokémon, if you're a pantheonfag.