If this was about just being popular & well known, Xykon would have won, because he's trendy (Vecna, like most on this list, hasn't done squat in quite a while now). That's not what these are about (as noted by the hate most popular characters normally receive). Oh, and I find it hilarious that you consider having a small write up in the 3E DMG (which he was in only because they made Greyhawk the standard campaign setting) to be more important than being on the cover of the 5E DMG.That's great and all. I'm a big fan of continuity gags, which are all over the place in 5E.
That doesn't make him popular or well-known, though. He's basically an in-joke for anyone who might recognize him. And, like any other in-joke, it's entirely meaningless to anyone who doesn't.