The phrase crème de la crème designates the best people in a group, or the best type of a particular thing.

It is a borrowing from the French phrase crème de la crème, literally cream of the cream—although the figurative uses of crème de la crème in French texts postdate the figurative uses in English texts.

The earliest occurrence that I have found of the phrase crème de la crème in a French text is a literal use—it is from Suite du Mémoire sur les Bœufs & les Vaches, concernant ce qui regarde les Vaches à lait, leur produit & la nourriture générale du Gros Bétail, published in Journal Œconomique, ou mémoires, notes et avis sur l’Agriculture, les Arts, le Commerce, & tout ce qui peut avoir rapport à la santé, ainsi qu’à la conservation & à l’augmentation des biens des Familles, &c. (Paris: Antoine Boudet) of October 1759 [page 448, column 1]:

Il en est du beurre comme du vin. Son excellence dépend d’abord de la nature du terroir qui a nourri les Vaches, comme celle du vin dépend du terroir où la vigne est plantée. Il y a ensuite le choix dans les différens laits, comme le choix dans les raisins. Il y a enfin la crême de la crême même, comme dans le vin la mere goutte ¹. C’est de cette premiere crême qu’on fait le beurre le plus excellent.
     translation:
The same goes for butter as for wine. Its excellence depends first on the nature of the soil that has nourished the cows, as that of wine depends on the soil where the vine is planted. There is then the choice among the different milks, like the choice among the grapes. There is finally the cream of the cream itself, like in wine the mother drop ¹. It is from this first cream that the most excellent butter is made.

¹ French mère goutte and English mother drop designate: juice which runs from a wine press before pressure is applied; hence, also: wine made from grape juice which has been collected in this way.

The earliest figurative uses of the phrase crème de la crème that I have found in English and French texts are as follows, in chronological order:

1-: From Letter to Lord Brougham on the Elective Franchise (London: John Smith, 1839), by ‘B. M.’ [Barclay de Mounteney, gentleman of the Privy Chamber to William IV]—as quoted in The Courier (London, England) of Monday 25^th February 1839 [No. 14,841, page 3, column 3]:

“That in common with all masses of men, the retailers have amongst them some foolish, silly, sycophantic members, subservient enough to play the willow to their patrons, and vain enough to imitate the most ridiculous fantasies of the most ridiculous of their superiors, I most willingly, as who would not, admit; but, in owning to this feature of the shopkeepers, it is only allowing what with equal veracity may be remarked of any other body of men, not omitting that choice body, yclept the exclusives, or as the facetious Madam Trollope styles them, la creme de la creme.”

2-: From a footnote to a poem entitled To My Dog, by ‘Hitcheepucksasassa’, published in the Army and Navy Chronicle (Washington, District of Columbia, USA) of Thursday 19^th March 1840 [Vol. 10, No. 12, page 190, column 2]:

Co-a-coo-chee—or Wild Cat—the greatest rascal among the rascally Floridian Indians, a sort of aborigine, “creme de la creme.”

3-: From De Clifford; Or, The Constant Man (Philadelphia: Lea and Blanchard, 1841), a novel by the British author and politician Robert Plumer Ward (1765-1846) [Vol. 2, page 250]:

“The question [is] whether grace, even the most exquisite, will alone raise a person into fashion who has no other pretension to it. Not only it will not (for how many very graceful persons do we not see neglected though looked at), and, on the other hand, how many personifications of clumsiness (large limbs, high shoulders, and enormous bon point), do we not behold in our best drawing-rooms? What exhibitions are there also of scraggy, flat, ill-formed machines, inrolled in, and constituting what is called, the very best company! These you will meet not only in the creme, but in the creme de la creme, as it has been called, of high society.”

4-: From the Brighton Gazette and Lewes Observer (Brighton, Sussex, England) of Thursday 29^th July 1841 [No. 1,067, page 2, column 3]:

We have heard a great deal said by the Whigs about the ability of Lord Edward Howard. Judging, however, from the speech delivered by his lordship the other day at Worthing, we should be disposed to set a lower estimate upon the noble lord’s talents—or at all events upon his discretion—than that taken by his political and partial friends. For example, we are told that his lordship said—
[extracts from Lord Edward Howard’s speech]
[…] But the crême de la crême is yet to come. Listen, gentle reader:—
[another extract from Lord Edward Howard’s speech].

5-: From Fashionable Life in Paris, published in The Morning Post (London, England) of Saturday 24^th September 1842 [No. 22,366, page 3, column 4]:

Amongst the literary men in Paris, Monsieur Eugene Sue ² is, perhaps, the man who has managed to establish himself with most eclât in circles of high fashion. It is not that he is of noble lineage, for his father and grandfather were doctors. However, the succês de salon of the author of “Mathilde” has somewhat intoxicated the gentleman, and now he gives himself the airs of choosing hypercritically his society, even amongst the creme de la creme, and he makes you understand that he is not prodigal of his company.

² This refers to the French author Eugène Sue (Marie-Joseph Sue – 1804-1857).

6-: From Parisian Fashions for November, published in The Morning Post (London, England) of Tuesday 1^st November 1842 [No. 22,398, page 3, column 4]:

The mournful event which has compelled the wearing of black has convinced the ladies how favourable it is to their charms. Dresses of black Cashmere, of crow-coloured soft Saxony cloth, are now called a costume de distinction; but the form of the sleeves, the disposition of the small buttons used by hundreds—of the cordonnets and gymp applied at every salient edge, sometimes starting from many points and converging in the ceinture, are objects requiring the utmost study, each feminine form requiring a new disposition. In Paris the severity of this toilette will render it more general than transcendently exquisite, for the belles incroyables chefs d’œuvres have been invented worthy of imitation by your own creme de la creme.

7-: From La Vie d’artiste (Paris: Hippolyte Souverain, 1843), by André Delrieu [Volume 2, Chapter XIX: Les salons de Vienne, dated Wednesday 25^th July 1838 – pages 119 & 120]:

D’abord le beau monde de Vienne se compose d’une fine fleur de l’aristocratie qu’on nomme la crême. Mais la crême elle-même n’est qu’un surnumériat ou un purgatoire dans l’expectative du monde extrà qu’on nomme la crême de la crême. […]
Dînez-vous place Michel, la maîtresse du logis vous tire à l’écart et vous dit avec mystère : Nous sommes la crême. Soupez-vous rue l’Évêque, une douairière, entre deux choucroûtes, trouvera le moyen de vous glisser à l’oreille ces paroles significatives : Nous sommes la crême de la crême. Allez-vous danser dans Léopoldstadt, un essaim de valseuses, en vous tournant la tête, s’écriera entre deux mesures de Launer ³ : Nous sommes la crême de la crême de la crême !
     translation:
First Vienna’s high society is composed of a flower of the aristocracy called the cream. But the cream itself is but supernumerariness or purgatory in expectation of the extra world called the cream of the cream. […]
Are you having dinner in Michael Square, the mistress of the house will take you aside and tell you secretively: We are the cream. Are you having supper in Bishop Street, a dowager, between two sauerkrauts, will find some means of whispering in your ear these significant words: We are the cream of the cream. Are you dancing in Leopoldstadt, a bevy of waltzing girls, turning your head, will cry out between two bars of Launer ³: We are the cream of the cream of the cream!

³ Here, Launer is a misspelling of Lanner: Joseph Lanner (1801-1843) was an Austrian dance-music composer and dance-orchestra conductor.

8-: From Les Enfants sans soucis, roman de mœurs (Paris: Hippolyte Souverain, 1843), by Eugène Deligny (1816-1881) [Vol. 1, page 25]:

Voici mademoiselle Clémence, ma fiancée et son brave homme de père, M. Landry, ex-fruitier et ancien de la vieille. Il a servi avec l’auteur de mes jours, il a veillé sur mon adolescence ; il m’a inculqué une foule de principes vertueux à grands coups de canne ; c’est la crême de la crême des honnêtes gens.
     translation:
This is mademoiselle Clémence, my fiancée and her honest man of a father, Mr. Landry, former fruiterer and one of the old brigade. He served with my noble progenitor, he kept a watchful eye on my adolescence; he caned loads of virtuous principles into me; he is the cream of the cream of honest people.

The earliest figurative use of the variant cream de la cream that I have found is from the Nottinghamshire Guardian (Nottingham, Nottinghamshire, England) of Friday 11^th February 1870 [No. 1,251 – Supplement: page 2, column 1]:

KITCHEN GARDEN SEEDS AND ROOTS.

Under the head “Finger-posts for purchasers” Mr. Shelley Hibberd occasionally gives capital selections of season plants.
The following, which we copy from the Floral World for the present month, is so thoroughly in accordance with our own experience that we have much pleasure in transmitting it to our columns. Let any one look upon this selection which contains the cream de la cream of kitchen garden vegetables, and then, taking up the “fat” catalogues which bewilder the purchaser, tell us the use of those interminable lists.