ditto can only transform and copy attacks. this doesnt mean that a ditto has the same skills in using the attacks, so no, ditto is not the strongest. there isnt a best pokemon, each one has advantages and disadvantages against one another, but some are better in certain areas. mew for example, is very illusive and hard to spot because of it special teleport and ability to transform into any other pokemon. mewtwo has very high physic( yea i think i spelled that wrong lol) abilities, and can control things, but still is not the strongest.