Mew was the original pokemon who could learn all the moves. I never heard Professor Oak say anything about Arceus or Smeargle, but I know that they can learn every move.
Of course its Arceus. He can learn every special move including his own, perish song, which kills Arceus and every other pokemon thats battling in three turns. Its Epic. :-/
mew becausearceus cant learn any punching moves (moves with the word punch in it) as he does not have any hands. mew can learn all tms, hms and any other move that is learned naturally by any other pokemon, even transform.