The the Penguin. Many fans have assumed that in the grand final battle of Batman's life, surely the Dark Knight's final foe will be the Joker. It turns out that's actually not true and Batman's ultimate enemy has always been Oswald Cobblepot, the Penguin.
Oswald Cobblepot is one of Batman's more civilized enemies. Instead of living in the sewers and eating whoever he can get his hands on or being obsessed with terrorizing people through fear, Oswald has more or less been happy to gather his power through mostly legal means. Oswald runs the Iceberg Lounge, which on the surface is an extremely popular nightclub. Only behind closed doors do the real shady dealings take place. While Oswald is absolutely a criminal, as far as Batman villains go, he's one of the much more traditional ones.
When the Batman Who Laughs escapes the Dark Multiverse, he has a nefarious plan to destroy the world, but this requires him to visit the Iceberg Lounge. While he's there he drops a bombshell of a revelation, as seen in The Batman Who Laughs #3 by Scott Snyder and Jock. The Batman Who Laughs explains to Penguin, as he's slaughtering all of Penguin's guards, that many people would assume Joker would be the final enemy that the Batman ever faces. But actually, it's Oswald Cobblepot himself that is the ultimate final boss for Batman. The Batman Who Laughs continues on by saying, "You grow this crime empire until he has to deal with you. On some worlds you win, get folks to turn on him. On a couple, he wins."
Penguin Builds An Empire To Rival Bruce Wayne's
This idea that Penguin eventually gathers enough legal power to defeat Batman has been shown in multiple different universes. In Geoff John's and Gary Frank's Batman: Earth One, the Penguin becomes the Mayor of Gotham City. While he does this through shady means, it is still a legal office that comes with a great deal of power and influence over the city. This also reflects Bruce Wayne's own struggle with his wealth. While Bruce has tried to save the city of Gotham through charities and other public services, almost all but giving away his money to help others, Penguin has done the exact opposite. Hoarding everything he has to grow in power to the point where, in some worlds, even Batman can't stop him.
On plenty of realities and earths, the Joker is the greatest villain Batman has ever faced and oftentimes the final villain.But as the Batman Who Laughs says, on the majority of alternate realities, it's the Penguin rather than the Joker who proves to be Batman's ultimate foe.
