Through a glass, darkly.

Barbosa: So what now, Jack Sparrow? Are we to be two immortals locked in an epic battle until Judgment Day and trumpets sound?
Jack Sparrow: Or you could surrender.

Superman. Lex Luthor.
Spider-Man. The Green Goblin.
Hal Jordan. Sinestro.
Captain America. The Red Skull.
The Flash. Professor Zoom.
Charles Xavier. Magneto.
Captain Marvel. Black Adam.
Reed Richards. Victor Von Doom.
Batman. The Joker.

Heroes are defined by their villains just as much as they are defined by their abilities or their secret identities. In each of the above pairings, it is hard to imagine that hero without the corresponding villain. But what makes a good archenemy?

In each of these examples, the common denominator is that the villain is a reflection of the hero. The reflection is usually twisted in some way, but it remains a reflection nonetheless.

Now, this reflection can come in many different styles. Sometimes the villain is an equal match for the hero with identical, or nearly identical, powers. They have a distinct and opposing personality and goals, but in terms of their abilities, they are very similar. Examples of this type of "mirror image" foe would be Captain Marvel/Black Adam, Wolverine/Sabretooth and Hal Jordan/Sinestro. What is interesting is that in these situations the heroes and villains often got their abilities from the same source. Heroes in this situation can easily look at their arch-foe and say, "There but for the grace of God, go I."

The next type of reflection is the negation. If the hero is physically strong, the enemy is physically weak, but intelligent. If the hero controls light, the villain controls darkness. Superman/Luthor and the Hulk/the Leader are two perfect examples of this, and incarnations of Sinestro where he uses the yellow ring make him this kind of enemy for Hal Jordan.

The third type is the rivalry where the heroes abilities have nothing to do with the villains, either in similarity or opposition. No, these are the mirrors where the hero has either an origin or philosophy which is similar to that of their foe. Magneto and Xavier are perhaps the best example of this. Both want a future for mutants that does not involve mankind destroying them, but their visions are in stark contrast to one another.

Captain America and the Red Skull were both created to be the symbol for the ideology that created them. Spider-Man and the Green Goblin are both byproducts of science gone wrong. Indeed, depending on which incarnation of the character we're dealing with, sometimes their abilities came from the same source.

And of course, the Batman is just as insane as his most vicious foe. His insanity makes him brilliant and a force for good, but the strength of his commitment goes beyond the actions that any sane man could consider.

Of course, there are arch-foes where the characters overlap into two or even all three of these categories. But there are very few who don't fit into any of them. And there is a reason for this.

I think that the key to a good arch-foe is the one who shows the hero what they could be. The best villain makes the hero question them self, even as the hero stops the villains plans. Even when you are not dealing with the archenemy of the hero, a hero's rouges gallery will often deal with many reflections of what the hero could have been. All of which works nicely to make the hero's struggle not merely a physical one, but a battle of morality and ideology. A superhero exists not just to punch the bad-guy in the face, but to inspire us to what we could be, and a good villain illustrates that struggle.