I think Batman probably has the highest IQ of his immediate circle. But there are others who are better/smarter in specific areas. Batman definitely doesn't know
everything, but when it comes to deductive reasoning, tactical ability and a very broad factual knowledge, he has very few peers. Especially the combination of all these faculties makes him near-genius level.
I think Bruce even excels in a lot of areas wherein others claim to be specialists... Arguably he's a better psychologist than Hugo Strange or Jonathan Crane, for instance.
The Riddler may have greater logical faculties than Bruce, but he lacks smarts in so many other fields (constantly allowing his own behaviour to trump him, a very narrow view of the world), that I'd hardly call him smarter than Batman. If anything, Batman's intelligence is highly practical, while Riddler's is almost purely theoretical. And theory only gets you so far...
It's been suggested that the Joker is a genius, but seeing how his brain is wired so utterly different from anyone elses, I think he shouldn't be compared to others.
I wouldn't say Ra's Al Ghul is
smarter than Bruce per se, but I think his unique perspective on things and centuries of experience have given him greater
insight. He's more philosophical, able to make further abstraction of concepts than Bruce is.
Lex Luthor might indeed be a lot smarter than Bruce. He's as much (if not more) of a tactical mastermind, he's very well-read and educated, on top of his immense scientific know-how. He's the only one I can think of that really equals or surpasses Bruce in more than just a few fields.
Interesting points made about Superman though... I've never really thought about how his brain works. I doubt the effects of our sun on his Kryptonian constitution actually make him smarter (or at least
able to be smarter) than ordinary humans... Sure, he can super-speed read, but does he process all the data like we do? Does he remember it? Does he understand it all?
He can read through all the major philosophical works of human civilisation, but does that mean he can grasp it all? Basically
anyone can
read a book, but that doesn't mean they understand it. Superman just reads faster. If he reads this loooooong list of the decimals of Pi, can he commit them all to memory?
On the other hand, in "Superman: the Movie", I never really got how exactly he "learned" all that info from "all the known galaxies" through the talkie-crystals on the flight from Krypton...

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