Usually, retcons (shortened form of "retroactive continuity") suck. All they seem to do is needlessly complicate characters or stories, or even worse, darken those same characters and stories that we previously admired or enjoyed. However, some retcons are necessary evils for the sake of at least the pretense of internal consistency when your setting functions on a sliding timescale that condenses 50+ years of stories since the characters first debuted into 10+ years within the narrative. And some retcons . . . actually justify the continued existence or relevance of characters (and stories) that would otherwise be incredibly dated or highly improbable. This is where we can discuss those retcons that we think worked.
Mine is from Mark Millar's Marvel Knights Spider-Man #1-12, which while happening in a Spider-Man comic has greater implications for the Marvel Universe as a whole. Many if not most of Marvel's most enduring and popular heroes debuted when the Cold War was still in full swing and nuclear war between America and the Soviet Union seemed dangerously possible, verging on inevitable in some or many people's minds. As such, many of those heroes were in turn opposed by villains who were operatives for the Soviet Union equipped with superhuman abilities or extremely advanced technology to even the odds against American superheroes. In 2004/2005, the period in which Marvel Knights Spider-Man #1-12 was published, the Cold War had been over for over a decade, thanks to the Soviet Union collapsing on itself in 1991 and thus rendering so many of the villains created in that era not only obsolete, but outdated and even anachronistic.
"So what does this have to do with Mark Millar's Spider-Man?" you might be asking me or yourself.
My answer would be, "Everything." Millar came up with a retcon tying together and updating all of these villains that by then seemed to have lost their luster or come to exist only so superheroes had someone to beat up in between more major threats. Specifically, supervillains were a response to superheroes, but a response deliberately engineered by corrupt corporate executives and politicians who saw the then-nascent superhero population as a threat to their own power and status, and created many of the early supervillains to keep superheroes too occupied to take on the executives and politicians for their greater crimes against society and humanity at large. How did this revelation come about? Because Norman Osborn, the mastermind of so much of Spider-Man's anguish, was one of those corrupt executives responsible for creating supervillains before he decided to become a supervillain himself, and through Mac Gargan/Scorpion (who later became Venom so he wouldn't be a punching bag for Spider-Man anymore), he revealed this dark truth behind the rise of the supervillain to Peter Parker.
Why did this retcon work? Because it updated the shadowy conspiracy behind the creation of many supervillains in the 1960s-1980s from Soviets/Communists, who were discredited and largely irrelevant by the 21st century, to something a lot more timely in an era that would see surging revelations about political and corporate corruption --- and how the two all too often worked hand-in-hand (or hand-in-glove) with each other. For a superhero setting taking place in contemporary times, it was quite believable that those endowed with abilities that could irrevocably change the world, for better or for worse, would have to be deliberately distracted from actually using those abilities to change the world, especially if their efforts could cut into the bottom lines of certain wealthy and politically powerful individuals. The best part of this retcon was that rather than completely and randomly altering the origins of various supervillains, it instead altered the context of those origins for the modern era. That, to me, was why it ultimately worked.
But that's enough out of me. What retcons do you all think worked?