What got me thinking was the 'Gamma World' episode (i think) of Worlds Mightiest Heroes.
The shorthand description of the Leader was that he was in intellect what the Hulk is to strength as a result of gamma radiation. Now as the Hulk gets stronger the madder he gets, what's the deal with the Leader? Does his intellect just keep growing and growing exponentially until one day he surpasses everyone on the planet? I dunno, I guess I've never really thought of it that way before.
Doom has it for being an 'all-rounder' but intellect is the Leaders whole schtik; it's his one, dedicated speciality. Plus, one could argue that while Doom is the most intelligent human supervillain, the Leader has a super-powered intellect and so must be smarter by definition.
It's certainly an interesting subject [and obviously one thats not going to be resolved on here short of Marvel actually coming out and saying who's the smartest!) but interesting nonetheless.
Imagine if all these guys teamed up against the smartest heroes - would they be able to cope/