The best is the enemy of the good.

[Il meglio, è l’inimico del bene.]

Voltaire (1694-1778) French writer [pseud. of Francois-Marie Arouet]

Letter (1743-06-18) to the Duc de Richelieu
    (Source)

A signature phrase of Voltaire's, attributed (by him) to a wise or sage Italian. His French translation is "Le mieux est l’ennemi du bien." Other mentions include an entry on "Art Dramatique" in his Philosophical Dictionary (1764), and the poem "La Bégueule" (1772).