Origin of "cream of the crop"
Solution 1:
I haven't heard it myself, although I have heard of "cream" being used to refer to as the best. After looking it up, I discovered that someone makes this statement:
Cream is the richest part of the milk and rises to the top. The noun ‘cream’ has been used to mean the best of any collective entity since the 16th century. ‘Cream of the market’ was an early expression using ‘cream’ figuratively and John Ray used the phrase ‘ the cream of the jest’ in his 1678 collection of English proverbs.
Those are pretty early quotes, but is that where "cream of the crop" originated from? Reading on:
‘Cream of the crop’ was undoubtedly adopted for its alliterative appeal and probably borrowed from the French ‘la crème de la crème,’ the cream of the cream, the best of the best, which was well-known in English by about 1800. But exactly when and where the ‘cream of the crop’ first came into use is unknown.
It probably came from French, although this is not absolutely definite.
Solution 2:
Cream is the part of milk that concentrates most of the fat. As such, it's often considered the best part of milk, and by extension the best part of anything. This is reinforced by the fact that the cream is what floats to the top. The word for cream can metaphorically mean the best part of something in many European languages. I don't think there's any etymological connection here, rather shared cultural referents (milk as a common foodstuff, valorisation of fat, positive connotation of top over bottom).
Cream of the crop started out meaning semi-literally best of the crop. The extension to mean the best of anything seems to have occurred in the second quarter of the 20th century.
There is an earlier expression, crème de la crème (often spelled creme de la creme), which is a borrowing from French (where it means, literally, cream of the cream). In both languages, the expression means metaphorically the best of the best, i.e. the very best. The expression crème de la crème is rather uncommon in French, where I think the use of crème to mean best has declined. The French expression may have had additional appeal in English because French evoked sophistication and the ruling class. In the 20th century, the use of creme de la creme declined (possibly due to the reduced exposure to and influence of French in English-speaking countries), though it has seen a recent revival.
Solution 3:
An organizational principle often stated by those at the top of an organization is, "The cream always rises to the top." (Those at the bottom say the organizational model is more like a cesspool, in which the largest chunks of fecal matter rise to the top.)