Idk, the episode when Mandarkk was introduced, Dexter was owned in almost every academic way possible by him(IQ test, free trivia questions, ...etc). It's unarguable that Mandarkk's intelligence was heavily nerfed for the sake of plot later on. Also, the episode when Dexter was distracted in a chess tournament by his mom from embarrassment to end up losing really doesn't help his case here. I know that I've already stated in previous posts that CN shows make debates awkward due to devastatingly inconsistent continuities. But the best Dexter(with out any enhancements from an invention) reaches 4th place at best.
Oh, I added characters for fun.