Genius-Level Intellect
Sure, the Joker is no dummy, but can he compete in the intelligence department with Nigma? After all, the Riddler is the guy who took control of Gotham City and turned it into a wasteland where only the intelligent were meant to survive in Scott Snyder and Greg Capullo’s Zero Year. How many can claim to have successfully done that?
If you think about it, the King of the Question Mark is also the original John Kramer, AKA Jigsaw. His contraptions and puzzles tested the brains and tenacity to survive of his adversaries in brutal fashion. Many faltered and met their demise at his hand, naturally, except for the pesky Batman who always has to be an overachiever and make others look bad in the process.
Nigma isn’t renowned as the smartest Bat villain for nothing. He has a sharp mind and even sharper knife; qualities that make him one of the most dangerous foes in DC Comics. In a battle of wits, there aren’t many who can compete with him. Once he sets his mind to something, you know you’re in for a hard time defeating him.