Guys, guys, settle down! It's offical! The true creator/ancestor of all Pokemon is actually Satoshi Tajiri!
If you think of it, this case is quite similar to the one true God (in most religions) and our first ancestors, the singular cellular DNA thingies! Why? Because fyi, Arceus's traits were actually based on Yahweh, the one true God in Jewish religion, while Mew was based on those single cell DNA thingies, from which all real world life forms evolved from. Also, Mew's Pokedex entries never stated Mew to be the ancestor of all Pokemon. No! It only stated that Mew contained every single DNA of each Pokemon and is believed by scientists to be the ancestor of all Pokemon, meaning that Mew being every Pokemon's ancestor is just a theory. Arceus could've just created Mew for the purpose of carrying every Pokemon's DNA, including Arceus's. Anyway, while Mew being the ancestor of all Pokemon is merely a theory and not a fact, Arceus's species is identified by the Pokedex as the Alpha Pokemon, meaning the first. The Original One.
Speaking of species, did you know that the Pokedex identifies Arceus as the Alpha Pokemon and Mew as the New Species Pokemon? Based on these names, which do you think came first? Alpha is the very first letter of the greek alphabet and is usually used to symbolize being the first and original one. Mew, on the other hand, is a New Species Pokemon probably because when Arceus created Mew, it meant the start of a new generation of species. A species called Pokemon.
C'mon, guys! This is hardly as hard as the 'Chicken or Egg' debate...