The general agreement I've found is that Mew was the first Pokemon Arceus created.

Arceus always messes with me when it comes to Pokémon. I mean, Arceus is a Pokémon, that's for sure, but if Mew is the genetic ancestor to all Pokémon, does that mean Mew is older than fucking universe? At some point after Arceus was born, he just floated up to Mew and said "Hey, do you mind if i create a universe for us to live in?"

I like Arceus, but he just fucks up the lore so much for me.