Suggested by Sonic Manaphy has some strong water type abilities even if he is completely overshadowed by just about every other water type legendary. That will still be enough to beat Ditto. Ditto isn’t really a combat type Pokemon. He can transform a lot and has access to many special moves but a copy will never beat the original. Ditto has some severe limitations as a result and won’t be able to claim victory here. Manaphy wins.