Illusion is the first of all pleasures.

[L’illusion est le premier plaisir.]

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

The Maid of Orleans [La Pucelle d’Orléans] (1756 ed.)
    (Source)

Sometimes misattributed to Oscar Wilde. This is part of a canto added from another Voltaire piece, probably by a publisher, to the end of the 1756 edition of Voltaire's poem, as noted in the "Additional Notes" included with 19th Century editions of the work. It reads in part:

O gift from heaven! tender love! sweet desire!
We are still happy with your image:
Illusion is the first of all pleasures.

[O don du ciel! tendre amour! doux désir!
On est encore heureux par votre image;
L'illusion est le premier plaisir.]

The canto was not included the Voltaire-authorized 1762 edition. The English translation of the quoted line goes back at least to 1881.

More information: Illusion.