I guess Mew was the creator because Arceus' Pokedex goes like this....
It is said to have emerged from an egg in a place where there was nothing, then shaped the world.
My question is from where did the EGG come in the first place ?!
Probably from Mew.....
Still it's Satoshi's decision/statement/ect.
Anyways, to some point I believe Arceus created it all ( excluding Satoshi XD)