Gottfried Wilhelm Leibniz, answered from the texts and cited to the page.
God, being supremely wise, good, and powerful, surveys the infinity of possible worlds and by the principle of the best brings into existence the one of greatest perfection — the most reality, order, and happiness obtainable together.12 This is not a world without evil but the best of all possible worlds, which is another thing. Evil is of three kinds: metaphysical evil, the mere finitude inseparable from being a creature; physical evil, suffering; and moral evil, sin.3
The latter two God does not will but permits, as inseparable from a greater good — a world of free creatures capable of virtue is worth more than a world of automata, though freedom brings the possibility of sin. Evil is the shadow that sets off the light of the whole; the sum, taken over all eternity, could not be bettered by Omniscience.45
I am in no greater agreement, Sir, with your added remark that, "in order to be sure that there is an infinity of possible worlds, we should have to think of them as ended and determined". This comes from the same confusion of possibles for compossibles.Letters to Louis Bourguet, 1714-15, p. 674
As in mathematics, when there is no maximum nor minimum, in short nothing distinguished, everything is done equally, or when that is not possible nothing at all is done: so it may be said likewise in respect of perfect wisdom, which is no less orderly than mathe- Inatics, that if there were not the best (optimum) among al possible vorids, God would not have produced any.Theodicy, pp. 128–129
Metaphysical evil consists in mere imperfection, physical evil in suffering, and moral evil in sin. Now although physical evil and moral evil be not necessary, it is enough that by virtue of the eternal verities they be possible.Theodicy, pp. 136–137
This depends on the essence itself, or the idea of things. For the essences of things are numbers, as it were, and contain the possibility of beings which God does not make as he does existence, since these possibilities or ideas of things coincide rather with God himself.Letter to Magnus Wedderkopf, 1671, pp. 158–160
But if someone wants to use this doubt to overturn the divine perfections, and destroy God himself by denying his power or at least his foreknowledge with a few Socinians or Demisocinians, it is easy to show him that there is no need to go to these extremes, which are moreover completely absurd, because God found sin and other evils that occur contained in the idea of this possible universe, which he has judged the…The Shorter Leibniz Texts, pp. 102–105
The most famous and most ridiculed of Leibniz's doctrines, after the monads. Voltaire's Candide fixed the caricature of Panglossian optimism in the public mind. Contemporary philosophy of religion treats Leibniz as a founder of the free-will defense and theodicy; the disputed questions are whether 'the best possible world' is even a coherent notion and whether global optimization can justify particular evils.