It can be all of them.
Bulbasaur is the first one listed in the national pokedex.
Mew is said by scientists to be the ancestor of Pokemon because its DNA is said to contain the genetic codes of all Pokémon. It was also the first Pokemon to receive trademark.
Rhydon was the first Pokemon ever created for the franchise and be encoded in the games.
And Arceus is known to be the first Pokemon in the universe, being born in nothingness and then shaping the Pokemon world.