I have tested this and I have seen that it is no doubtedly Roy's Sword Slash, fully charged. It's so powerful that it does damage to him!!! On Practice Mode, I got 999% health to Mario, then hit him with the fully charged homerun bat. Then hit him with Roy's Slash. Roy's Slash didn't even leave that wind thing, the thing which comes out of the side, when you K.O. them out of the side.So Roy's Slash is the most powerful move.