Which Pokémon was the first one listed in the National Pokédex?

Bulbasaur is the first Pokémon listed in the National Pokédex.

Bulbasaur occupies National Pokédex number 0001 and is a dual Grass- and Poison-type Pokémon. In Pokémon Red, Green, Blue, and their remakes, it is one of the three original starter choices offered to players at the beginning of the adventure.

Its design combines a small amphibian-like body with a plant bulb growing on its back. The bulb is planted at birth and grows as Bulbasaur develops, linking the species to its Grass typing and later evolutions, Ivysaur and Venusaur.

Pikachu is more famous as the franchise mascot, while Mew is the final entry in the original Pokédex at number 0151. Neither therefore comes first in the National Pokédex numbering.