Capitalism is the astounding belief that the most wickedest of men will do the most wickedest of things for the greatest good of everyone.

John Maynard Keynes (1883-1946) English economist
(Attributed)

Commonly attributed to Keynes, it is not found in his works. A more likely variant is "Capitalism is the extraordinary belief that the nastiest of men for the nastiest of motives will somehow work for the benefit of all," but this, too, is not established in Keynes' works. Also sometimes found attributed (without citation) to John Kenneth Galbraith.

George Schuster, Christianity and Human Relations in Industry (1951) quotes (uncited) Keynes referring to capitalism as "the astonishing belief that the nastiest motives of the nastiest men somehow or other work for the best results in the best of all possible worlds."

E. A. G. Robinson was a close colleague of Galbraith, and in his book Monopoly (1941), he said, "The great merit of the capitalist system, it has been said, is that it succeeds in using the nastiest motives of nasty people for the ultimate benefit of society." Robinson may be quoting Galbraith, or Galbraith may have anecdotally quoted Robinson.

More detailed discussion of this here, here, and here.