This is kind of a mind fuck. Because if Mew is the ancestor of all Pokemon, does that make it the ancestor of Arceus too? And if not, does that mean Arceus isn't a Pokemon?

And taking the genesis of Pokemon into account, does that make Rhydon the first Pokemon ever since Pokemon may not have existed without the design of Rhydon?

That being said, lore wise I'd have to go with Arceus since it stands to reason that Arceus created the Pokemon universe and then Mew would be the ancestor of all subsequent Pokemon.