He Has Successfully Deduced Batman’s Real Identity
Yes, this isn’t a novelty, since the likes of Bane and Ra’s al Ghul have also deduced that Bruce Wayne is Batman. However, the Riddler is the one who has used the knowledge to his benefit by keeping the Dark Knight on his toes.
By possessing knowledge of Bruce’s nocturnal activities, he holds leverage over the Caped Crusader and could easily reveal to the world who the man behind the mask is. Instead, he keeps his cards close to his chest and plays the long game.
While a regular street thug would’ve sold the secret off to the highest bidder by now, Nigma realizes that this information is much more valuable than gold or silver, choosing to keep it hush-hush. It’s his get-out-jail-free card, if need be, and he can utilize it as a bargaining chip when he needs to. It’s deviously brilliant.
He’s A Master Manipulator
The Joker is a fan of physical persuasion. If he wants you to do something, it’s best to adhere to what he says, or he’ll slice your throat – or murder your entire family. There’s no negotiation or niceties exchanged when you’re dealing with the psychopathic clown.
The Riddler has what you can term social capital. He’s able to get people to bend to his whim without even having to resort to violence at the best of times. In fact, if he wasn’t adamant about becoming a dastardly villain, he would’ve made a mighty fine insurance salesman, selling you a whole bunch of crap that you’ll never need or use in ten lifetimes.
Over the decades, we’ve seen numerous instances where Nigma has convinced both heroes and villains to do his bidding. He doesn’t rely on fast talking or underhanded mind control techniques like some of the other knaves; no, he uses his psychological superiority to convince the person that they’re doing the right thing by listening to him. He’s like a politician – only smarter and more useful.