“Hyper-selectionism has been with us for a long time in various guises; for it represents the late nineteenth century's scientific version of the myth of natural harmony—all is for the best in the best of all possible worlds (all structures well designed for a definite purpose in this case). It is, indeed, the vision of foolish Dr. Pangloss, so vividly satirized by Voltaire in Candide—the world is not necessarily good, but it is the best we could possibly have.”

Introduction, p. 1.
The Best of All Possible Worlds (2006)

"Times Must Change" in Ability # 179 (20 March 1966) http://www.able.org/about/l-ron-hubbard/articles/times-must-change.php.

Appearance and Reality, preface http://books.google.com/books?id=EtgtAAAAYAAJ&q=%22of+optimism+I+have+said+that+The+world+is+the+best+of+all+possible+worlds+and+everything+in+it+is+a+necessary+evil%22&pg=PR14#v=onepage (1893).

This is actually James Branch Cabell from The Silver Stallion (1926)
Misattributed

This is derived from a statement of James Branch Cabell, in The Silver Stallion (1926) : The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true.
Misattributed
Variant: The optimist thinks this is the best of all possible worlds. The pessimist fears it is true.

Coth, in Book Four : Coth at Porutsa, Ch. XXVI : The Realist in Defeat
Source: The Silver Stallion (1926)
Context: Yet creeds mean very little... The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true. So I elect for neither label.

The Almost Perfect State (1921)
Context: The best good that you can possibly achieve is not good enough if you have to strain yourself all the time to reach it. A thing is only worth doing, and doing again and again, if you can do it rather easily, and get some joy out of it.
Do the best you can, without straining yourself too much and too continuously, and leave the rest to God. If you strain yourself too much you'll have to ask God to patch you up. And for all you know, patching you up may take time that it was planned to use some other way.
BUT... overstrain yourself now and then. For this reason: The things you create easily and joyously will not continue to come easily and joyously unless you yourself are getting bigger all the time. And when you overstrain yourself you are assisting in the creation of a new self — if you get what we mean.

Attributed by Sir George Schuster, Christianity and human relations in industry (1951), p. 109
Recent variant: Capitalism is the astounding belief that the most wickedest of men will do the most wickedest of things for the greatest good of everyone.
As quoted in Moving Forward: Programme for a Participatory Economy (2000) by Michael Albert, p. 128
Attributed

"The Sealed Treasure" (1960), p. 60
It All Adds Up (1994)