It is, perhaps, the most visually arresting character DC ever created.
The Composite Superman.
DC just released a statue of him and, as a result, the internet is abuzz by the unsilverized saying "What th--?!" I always assumed everyone knew who the Composite Superman was; guess not. The topic's been covered beautifully elsewhere, so here I'll try to gather some of that together for the curious and nostalgic.
In short, he was Joe Meach, who ruined his high-diving career when Superman had to save him during some jackass stunt (World's Finest #142, 1964). Superman kindly gave Joe a job as janitor at the Superman Museum (rank has its privileges, you know), but Joe was bitter and resentful. Through a classic silver age mishap, Joe acquired the powers of the entire Silver Age Legion of Superheroes, remade himself as the Composite Superman and set out to humiliate the World's Finest. Humiliate them he did, but his powers (and his memory of them) faded before he got around to finally destroying Batman and Superman. In a comeback story, a wicked alien manipulated events to recreate Joe as the Composite Superman, a tale that managed to redeem Joe in the end. No one could do a better job of summarizing the plot and meaning of the story than this synopsis.
The impact of the Composite Superman on the minds of the young was enormous. As weird as the half-Batman/Superman was, the green skin (which symbolized his continual use of the 12th level intelligence of Brainaic 5) put it completely over the top. Be forewarned; once you've seen this page of him using the powers of the lamest Legionnaires to defeat Batman and Superman simultaneously, the image will be burnt into your mind's eye forever. Of course, Batman and Superman stood no chance against him. Zero.
After his two appearances, Joe himself never appeared again (he dies at the end of the second story and Superman takes steps to ensure that another Composite Superman cannot be created). But so strong is his "iconic resonance" that visual and narrative references to him keep turning up.
Since the Composite Superman's power came from the Legion, it was only fitting that a version of him turned up there (Legionnaires #25). Shape-shifting Durlans can copy only shapes and appearances, not powers...except for the "Composite Man", a Durlan who fought the Legion by copying all their powers. Shudder!
The modern inheritor of World's Finest, the Batman/Superman title (issue #6) , gave us another reference to the Compster, when the tiny Toyman creates a giant Composite Superman/Batman rocket for reasons no sane person wants to discuss.
Young Justice did justice to old Joe Meach when they fought Craydl, the assistant of Impulse's evil twin, Inertia (don't ask; it's a Flash thing). Craydl, an artificial intelligence composed of green technoplasmic goo, uploaded Robin and Superboy's DNA into itself for copying, but only got halfway through, resulting in the Composite Superboy, brilliantly customized here. Certainly it was the wittiest reference to the original character!
Despite being dead, Joe Meach seems to have his own blog (as previously noted by Progressive Ruin) but apparently getting hit twice by lightning has not imbued him with superblogging-power. He even showed up (it seemed) in an episode of Justice League Unlimited, a cameo that stunned watchers whether they knew what it was or not!
The Composite Superman was undefeatable, except by his own failings. The very idea of the Composite Superman is like that, too; it's still with us, butso powerful (or just wacky!) that it can only express itself in short glimpses.