Cream of the crop
The best of anything and cream has been used in this way since the 16th century. It is difficult to pinpoint a date for this expression because cream has been used in any context to signify the best e.g. the cream of swordsmen, the cream of archers etc. The French version, which is often heard in English, is crème de la crème or cream of the cream, which also dates from about the 16th century.