Before level upgrades, "items", etc... and assuming none of them have templates.
Reed Richards (and possibly Dr Strange): 18.
Tony Stark: 17-18
Peter Parker: 15 - 16
I have to believe all three of them started at 18, or higher in editions that allow a bonus in an ability of their choice for humans.
Using only straight 3d6 to generate a random person, (with rounding and possible typos) gives:
18 = 99.5% below them, with 0.5% equal and 0% above
17 = 98.2% below them, with 1.4% equal and 0.5% above
16 = 95.4% below them, with 2.8% equal and 1.8% above
15 = 90.7% below them, with 4.6% equal and 4.6% above
So, before leveling, 13,000 people born in the US in each year around 1940 would have had an 18 base. Reed and Stark were already among the top of their respective years before they did most of their leveling.
About 19,000 each year in the mid-1950s would have had an 18 before leveling. For Parker to have a 15, it would mean that not only would he not be winning any national or regional IQ challenges for high school students, but that there would be a decent sized group of people in his own high school's senior class as smart or smarter than he was.