In my point of view , the most powerful pokemon is :
It may sound weird , but in fact Mew is one of the 2 Pokemon (itself and ditto) . So it can transform itself into ANY OTHER POKEMON (including Arceus) and possess the powers of that Pokemon.
Arceus created the universe , fine , But Mew can effortlessly make some new universes with its powers (of Arceus) and can destroy this one at will.
It can even turn into Dialga then , reverse the flow of time , reach the point of time even before Arceus was born , destroy its egg , AND END THE DEBATE.
It can make Giratina come back from the distortion world , Make Groudon and Kyogre lovers , Rayquaza and Ekans soulmates , all in the parallel universes. (SOUNDS ABSURD :p)
IGN - oneandonly_SOURISH