Major Spoilers Question of the Day: Dark Mirror Edition

In the 1940s, every superhero had a sidekick that followed him around.  By the 1960s, it seemed everyone had a utility belt, and by the end of the Bronze Age, every bad guy worth his salt had an arch-nemesis whose evil was a perfect counterpart for their heroism.  Really good heroes, like Superman and Spider-Man, got more than one, balancing different aspects of their character, while Batman had at least five, depending how you count.  Regular podcast listeners will recall me complaining about overuse of the “Dark Mirror” trope, as it not only leads to overexposure of the likes of Venom, The Joker and Magneto, it makes certain fans see stories WITHOUT them as somehow insignificant.  After all, if Batman isn’t fighting his most terrible foe, how bad can things really be?  Still, there are a number of such characters whom I really enjoy, not the least of which being Darkwing Duck’s nemesis, the eeeeevil Negaduck, proving that even the most hackneyed bit of storytelling can be awesome with enough Jim Cummings, and leading us to today’s Earth-3 query…

The MS-QOTD (pronounced, as always, “misquoted”) felt the need to balance out yeterday’s Carnage hatred, asking: Who is the best example of the evil twin /dark mirror archetype in fiction?