The amazingly simple answer is this:
Most of it was created by a man named Jack Kirby. Nobody helped him. He sat at his drawing board and created, wrote, and illustrated almost every superhero at the company that came to be known as Marvel Comics. He created (with no help from anyone else):
The Fantastic Four (which did contain a re-imagined version of The Human Torch which was created by Carl Burgos).
The Incredible Hulk.
The Mighty Thor.
Ant Man (who later became Giant Man).
Sgt. Fury/Nick Fury.
The Silver Surfer.
Captain America (co-created with Joe Simon).
And all of the associated characters that went along with each of those main characters and titles.
It could even be argued that he created the Marvel version of Daredevil. The character's name was stolen from Lev Gleason (a company that had gone out of business). But the character images and concept was by Jack Kirby. The first issue and its origin story was written and illustrated by Bill Everett.
It's said that Kirby first suggested a character named Spider-Man, but his concept was rejected.
The other major creator at Marvel was Steve Ditko. He created, in those early days, the few things that Kirby did not. These were:
Dr. Strange. Again...no one helped Steve Ditko create Doctor Strange. He did it on his own with no help whatsoever from anyone.
Steve Ditko also revamped the character Iron Man when he was briefly assigned the comic when Kirby was too busy to do it. In the very brief time Ditko was there, he created Iron Man's sleek, re-imagined gold and red armor. This is the iconic Iron Man that has remained largely unchanged since that time (well over 50 years ago).
And, of course, Steve Ditko created all of the associated characters who surrounded both his Spider-Man and Doctor Strange creations.
There is no "co-creator" for these men. Their ideas were basically stolen from them. Billions of dollars of profit have been generated from their works. If we can't properly credit them as the sole creators of that work, at least the party that now controls those properties can do the right thing and acknowledge that they have rights to a respectable portion of the vast sums of income generated from their work.