I'm going with Arceus. He's the creator of the Pokemon World, which includes Mew. Here's my theory: Arceus created Mew first, and Mew somehow gives birth or 'evolves' into the Pokemon we know today. (By evolve, I mean the Charles Darwin kind of evolve, not the Pokemon kind of evolve.) Or something like that.
And as for the Arceus egg theory, I'm guessing that some unmentioned supreme being created the egg, and Arceus hatched out of it.
That supreme being is who all know as Satoshi Tajiri