The question is being asked with no context, and therefore, we must assume that it is being asked from a real world perspective, and not an in game continuity perspective.
Real world wise, Rhydon was the first Pokemon created and programmed into the original Red and Green games, and is therefore the first Pokemon.
However, in the context of in-game continuity, you need to do a little shenanigans.
Mew was said to be the first Pokemon, and the ancestor of all Pokemon, but then, in generation 4, they created Arceus, who is the Pokemon that created everything, including Pokemon, so that means that it also created Mew, which means that Mew is not the first Pokemon anymore. it has been retconned. To claim Mew is the first Pokemon is to claim that Arceus is not a Pokemon.
>Mew is out
Bulbasaur is only the first Pokemon in terms of the index number in the Pokedex, and is not truly "the first Pokemon" it's not like it's the first Pokemon ever to be discovered by humans, simply the first one they cataloged when making the Pokedex. It's index number is just #001. being at the top of a list of things doesn't mean that it predates the other things on the list.
SO
from a real world stand point, Rhydon is the first Pokemon
From an in-game standpoint, Arceus is the first Pokemon, Mew's firstness having been retconned.
It's science, bitch.
