First, one must distinguish between monarchy and aristocracy, as the two were often in conflict. Lords and nobles often saw it in their interest to have a weak king. And while Aquinas may prescribe monarchy as the best form of government, Aristotle was more sober in his assessment of best forms of government. While some people (esp. those in the Middle East) might have temperaments more conducive to monarchy, other people, he felt, were better served by aristocracy / oligarchy or politeia / democracy. The best form of government was relative to various ethnicities and their ancestral and tribal histories.
Second, Red is correct that a people cannot select a new aristocracy de novo. The European aristocracy, as it evolved out of the Middle Ages, was a product of the warrior caste. The early aristocracy ruled because they were able to rule. This authority in time became reflected in law. To bring about conditions for an aristocracy in the U.S. today, the U.S. would have to descend into complete chaos.
Third, regarding meritocracy, the ancient aristocracy was not opposed to meritocracy, but thought it was the epitome of merit. Echoing Nietzsche, bonus not only meant good, but also noble with its martial implications. Aristotle combined all these qualities, arguing that the moral were typically noble and even good looking. Both Plato and Aristotle were concerned with good breeding (eugenics), something which continues into the Middle Ages with gentilesse (refinement and courtesy resulting from good breeding).
There is some evidence that the nobles were the fittest. For instance, Gregory Cochran and Henry Harpending, in The 10,000 Year Explosion (pg. 104), write:
Gregory Clark, in A Farewell to Alms, shows that in medieval England the richest members of society had approximately twice the number of surviving offspring as the poorest. The bottom of society did not reproduce itself, with the result that, after a millennium or so, nearly everyone was descended from the wealthy classes.
Interestingly, although we are nearly all thus descendants of the Medieval upper classes, nearly the exact oppose phenomenon is occurring today: Dysgenics / Idiocracy.
Finally, let’s not forget that there was a racial element to aristocracy in Europe. “Blue bloods,” as they were called in Southern Europe, were supposed to be fair skinned (of Germanic, or maybe Celtic, ancestry) so that one could see the blue veins through their pale skin (unlike the dusky skin of the Moors, etc.). Throughout Europe the aristocracy seems to have been fairer skinned than the lower classes, as noted by the “fair princess” of fairy tales. In other words, fair looks denoted an aristocratic or upper-class background. Swarthy looks denoted a lower-class background.
Update: Richard Spencer comments here.