The world we live in is not the best possible world,
but it is the best way to the best possible world. In other words, the lessons learned in this world will eventually produce a world of a quality which could not exist otherwise.
I.e.
- Without free will love is meaningless, therefore free will is necessary for the best possible world.
- Free will by definition can be abused, producing something less than the best possible world.
- Through God's patient guidance, and by experiencing first hand the consequences of sin, free-willed creatures will learn to never again choose sin.
- The best possible world, where free will and sinlessness coexist, will be attained.