The approach you're taking with all these characters is, again, circular. You're assuming that the ranger archetype needs to have a favored enemy, then looking at whomever these characters happened to oppose and calling that their favored enemy. But every hero in adventure stories opposes someone. If you assumed that wizards had to have a favored enemy you could do exactly the same exercise for Merlin, Gandalf, and Harry Potter.
I suppose you could. It would be difficult for me to identify an FE for Merlin, however, unless it was "Sorceresses named Morgana".
Remember, this was
your exercise. You identified these characters as Rangers and then claimed that a Favored Enemy could not be found for them. My response was that if I were to attempt to build these characters as Rangers it would actually be quite easy to find an FE for them. I could just as easily build them as some other character class. If I were to build them as Wizards I wouldn't need to identify their FE because that has never been a class feature of the Wizard in any edition of D&D I know of.
My point here is that none of these ranger characters are distinguished by their hatred of a particular enemy any more than their non-ranger allies. You've said that hunting these enemies is the ranger's raison d'etre. But Artemis doesn't exist to hunt men. Natty Bumppo is an ally of the Indians. And Aragorn is not a more driven or better "Servants of the Enemy" fighter than Boromir or Gimli. If you were writing a new game with a class to model these characters, ignorant of the conventions of D&D, you would not think to yourself, "Okay, key to all these characters is their racial (and/or gender) hatred, so I'd better write that as an ability." Not the way you could look at wizard characters and say, "Oh, yeah, spells", or barbarian characters and say, "Berserker rage, easy, done".
Artemis exists to protect the wild sanctuary of the gods as she protects her own virginity. The natural enemy of such efforts is the mortal interloper who, unwittingly or no, penetrates the boundary of her privacy. If Artemis loves those she transforms into a stag and hunts to their death, she has a strange way of showing it.
Nathaniel Bumppo may be a romanticized ally of the Indians, but was Daniel Boone?
Gimli may have been as good against orcs as Aragorn, but remember he's a dwarf. It says right in the 5E PH that dwarves
hate orcs. It's a racial trait that's been in the game since
Chainmail. I'd actually like to see a version of FE given as a racial trait to dwarves, elves, etc. Now against a Nazgul, I think Aragorn might have had an edge over Gimli.
As for Boromir, I hardly think attempting to seize the ring and use it is proving yourself effective against Servants of the Enemy. Maybe if he had studied the stratagems and designs of Sauron more closely he might not have been ensnared in this trap.
Generally, literary characters are not well represented by D&D. They tend to have the features of many classes and are difficult to define in game terms.
And if you do not think these characters are rangers, who do you think is a ranger? Woodcrafty characters like these, I would argue, are who define the ranger class in most players' minds. I honestly can't think of a character who would define the class the way you describe. George R. R. Martin's Night's Watch is probably closest, but they of course postdate D&D rangers by decades and are probably based on them rather than the other way around.
How about Beowulf? He tracks Grendel out onto the heath and crosses the liminal boundary of the surface of the lake to destroy the monsters. The hero's journey often take a similar form, thus making the archetype widely applicable.
Also, many of your proposed favored enemies are not legal choices in D&D and would be OP compared to the regular options if they were. I mean, "Servants of the Enemy"? That's... the whole Monster Manual, in Middle-Earth.
You're accusing me of making Aragorn OP? This is
Aragorn we're talking about. Besides there are lots of monsters in Middle-earth that have nothing to do with Sauron.
There's the Balrog of Moria. I don't remember Aragorn being particularly good against him.
Smaug had no allegiance to Sauron, and Aragorn is never depicted as a dragon-slayer.
There are many things that are beyond him.
And, like I said, the rules of D&D come with their own assumptions about the game-world and don't model particular works of literature very well. To set a campaign in the lands of Middle-earth would require a bit of tweaking, don't you think? Wasn't that the type of thought experiment you were engaging in when you brought up all those literary characters as examples of Rangers?