One possible answer:
I suppose your thinking is that it is suffering and sin that make this world less than perfect. But then your question makes sense only if the best possible worlds contain no sin or suffering. And is that true? Maybe the best worlds contain free creatures some of whom sometimes do what is wrong. Indeed, maybe the best worlds contain a scenario very like the Christian story.
Think about it: The first being of the universe, perfect in goodness, power and knowledge, creates free creatures. These free creatures turn their backs on him, rebel against him and get involved in sin and evil. Rather than treat them as some ancient potentate might — e.g., having them boiled in oil — God responds by sending his son into the world to suffer and die so that human beings might once more be in a right relationship to God. God himself undergoes the enormous suffering involved in seeing his son mocked, ridiculed, beaten and crucified. And all this for the sake of these sinful creatures.
I’d say a world in which this story is true would be a truly magnificent possible world. It would be so good that no world could be appreciably better. But then the best worlds contain sin and suffering.
The entire dialogue is well worth a read. It’s between Gary Gutting and Alvin Plantinga, an emeritus professor of philosophy at the University of Notre Dame, a former president of both the Society of Christian Philosophers and the American Philosophical Association, and the author of Where the Conflict Really Lies: Science, Religion, and Naturalism.