I've seen this thread somewhere before.
Ya, having Deja Vu here, think this thread's been done 4 times just since I joined the Hype, not sure how many times before that lol.
As for Forge, I wouldn't count him even in the top 20. Forge can make anything, but most of the time he doesn't understand how it works. You could tell him to build you a gun, and he would, but he'd have no clue as to how the mechanics of it operate. Xavier has probably touched some very intelligent minds, and seems very well learned. Magneto has constructed plenty of high tech devices, and I'm not quite so sure on Mandarin. I know who Mandarin is and have read stories with him in it, but am far from an expert on him.
I'd go along with everyone and put Reed at the top, and then Doom, Pym at third, Stark 4th, Banner 5th, Leader 6th, Cho 7th, Beast 8th, and can't think of a good last 2 to round it out.
Now that's just Marvel Earth, if we count the entire Marvel universe then many of those guys get bumped down, and quickly. There are more advanced civilizations that an average citizen there would be a genius here. Then there are guys like Thanos who is just leaps and bounds ahead of Earth heroes on intelligence. Galactus probably has incredible intelligence as he's basically seen it all. Then there are others who have seen a lot, but don't put as much stock in technology, so while much more intelligent than most, they aren't surrounded by tech.