Joker is smarter wisdom and creative wise in Arkham Asylum the doctor described him as being an evolved form of man who sees the world differently than we do!
Webbing, Spider Tracers, Electricity detector, creating a cure for the lizard on the fly, building an armor from the webbing formular, building an alarm system based on his spider sense, building a tracking device for his camera that he's always centered, binding hydrofluoric acid to his webbing > Joker gas
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“I once had an entire race killed just to listen to the rattling of their dried bones as I waded through them.” —Volrath
Questions like this are always inherently flawed, as the question is actually asking more than what is covered by the word "smarter"; Joker has the edge in inherent, intellectual power, and certainly in cunning and planning. Peter has a much more natural bend to science and relatively applied intellectual use of scientific knowledge and principles. It is misleading to ask who is "smarter" -- each has their areas of emphasis.
But like I also indicated, if this is about raw intellectual power (including intuition, applied knowledge, creativity, insight, and general potential), I'd say the Joker just edges out Peter, who isn't always the most cunning or astute of individuals, despite his natural talent for "exotic" tech/applied sciences.
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Yeah, when it comes to tech geek stuff, Spidey has Joker cold.
The "different intelligences" argument is a valid one, though. For example, Cap isn't as intelligent as Spidey in technology. But, Spidey isn't half the leader or war specialist Cap is.
So who's smarter?
When talking about intelligence, people tend to think in terms of textbook education/technology, but there's many ways of being smart.. Even something as seemingly simple as social savy is a kind of intelligence...
Cap was trained to be a leader and has more experience due to his service in wars and being a full time Avenger. Learned tactics and experience does not equate into IQ points or being smarter. And Cap isn't in this thread anyway.
Spidey's brain power is more than anything Joker can muster. Spidey has shown that throughout his history. Joker can come up with elaborate, confusing plans and concoct his Joker venom. Spidey's artificial webbing is more impressive than Joker venom imo. It's something scientists can't even duplicate to this day.
Also, being psychotic and coming up with off the wall plans which confound most everyone does not make Joker a genius, just nuts. Though his brilliance does shine through from time to time.
Give Spidey days/weeks to plan something and he'll confound Marvel's best, at least for a time.