"Stark created Ultron."
Eeeh...don't you mean Ant-Man? Because I'm pretty sure Ant-Man is the one who created Ultron, unless you're talking about the new Avengers movie they're making where they change it to Stark.
Yes, Reed Richards is the smartest character in the Marvel Universe, that is true, and I never said he wasn't, I said Spider-Man was one of the smartest along with characters like Richards.
"Even in his old age, he still can outsmart the best of them" maybe part of that is his old age, or his experience. Really, the reason Spider-Man seems of lesser intellect is because he is younger.
"Could you imagine what it would be like if someone like the Joker actually got serious", yes, he would lose his greatest advantage which is his unpredictability (an advantage Carnage also has in addition to the shapeshifting and the super strength and the wall crawling and the stretching and all that).
All those feats of intellect you pointed out for the other heroes are true, however, you neglected to point out why Spider-Man does not rank among them. Here are some of his feats of intelligence:
Inventing the web shooters. They may not be Iron Man armors but hey, Tony has ten years on Spider-Man, and they are very impressive machines even if they look small and simple (hell, the ultimate nullifier, Richard's greatest machine looks very small and simple too, doesn't it?)
Created an EMP in seconds in the middle of a battle.
Cured a flesh eating disease by himself.
Ant-Man (you know, the creator of that awesome Ultron you just mentioned), claimed that Spider-Man was more intelligent than he is.
Took an intelligence test that Reed Richards took at Peter's age and they got the same score.
Hacked into Tony Stark's security systems.