Leibniz’s solution of the problem of evil, like most of his other popular doctrines, is logically possible, but not very convincing. A Manichaean might retort that this is the worst of all possible worlds, in which the good things that exist serve only to heighten the evils. The world, he might say, was created by a wicked demiurge, who allowed free will, which is good, in order to make sure of sin, which is bad, and of which the evil outweighs the good of free will. The demiurge, he might continue, created some virtuous men, in order that they might be punished by the wicked; for the punishment of the virtuous is so great an evil that it makes the world worse than if no good men existed. I am not advocating this opinion, which I consider fantastic; I am only saying that it is no more fantastic than Leibniz’s theory. People wish to think the universe good, and will be lenient to bad arguments proving that it is so, while bad arguments proving that it is bad are closely scanned. In fact, of course, the world is partly good and partly bad, and no “problem of evil” arises unless this obvious fact is denied.
--Bertrand Russell, A History of Western Philosophy, Simon & Schuster (1972), p. 590
Personally, I think the problem of evil may well be soluble, but I seriously doubt that Leibniz's "Best of all Possible Worlds" solution makes the cut.