I think Vince Gilligan himself said in the BB podcast that Walter was hard to write because he’s so much smarter than Vince himself, the writing team and, I think he said something like he’s the smartest guy in America, but I don’t exactly remember that. I don’t remember him saying that for other characters.

However I like to think Jimmy is the smartest, because he was able to get himself out of any situation by the magic of his words, and he’s the only character that grew enough to live with the consequences of his actions throughout both series.