Mewtwo anime wise, and Deoxys game wise...
Mewtwo-It's freaking Mewtwo, do I need to explain why he's the strongest?
Deoxys-I honestly think that an Attack Form or Speed For Deoxys with a bit of luck could 1HKO Arceus. (Ex: Atk Deoxys uses Psycic on a Posion Arceus and Deoxys get's a critical hit.) And I do realize that someone will come along, and list the IVs, EVs, Base Stats, and Nature of an Arceus that would take on Deoxys...