I followed that childhood by being a dealer in rare comic books for most of my adult life, stopping only when I was 38 years old.
So I saw a shitload of great old comic books in my day. Always my favorites were the creations of Jack Kirby and Steve Ditko at Marvel Comics. Kirby and Ditko did it all, and all by themselves (no matter what bullshit lies you may hear).
People talk about how rare and difficult various books are to locate. When it comes to early Marvels, almost all of them are hard to find in near mint or mint because of the low-grade cover stock they used which resulted in what was to be called "Marvel chipping" in later years. That is, the covers became brittle and chipped away at the edges as time passed. You do sometimes find them in perfect shape, but this is extremely rare.
But that's not what I'm here to talk about. What I want to talk about is the general rarity and availability of the various key issues in any grade at all. I always noticed that some books were very hard to find in any grade whatsoever, while others were relatively easy to locate. This was due, of course, to the number of issues that sold on the stands and which were subsequently available to be purchased as collectibles.
So, I'm going to list the key early Marvel books in order of how rare the key books were for me to find and purchase. Keep in mind---that this is subjective. The results were how hard the books were for ME to find and purchase. Other folk in other parts of the nation may have gotten different results.
The single rarest early Marvel superhero book was the first one:
Fantastic Four #1. In all of my years of experience of dealing old comics, I don't think I ever had but six of these in my stock. That's even counting my dad's days of dealing old books when that particular issue was only a few years off the stands. In our neck of the woods (the South), it was, and remains, an almost impossible find.
Next on the list would be Tales to Astonish #27, the first appearance of Ant Man. Now, when this story was first published it was just part of the general stock of monster/sf stories written and illustrated by Jack Kirby. But when Kirby was ordered to create more superheroes to capitalize on the DC superhero revival going on there, he reached back a year or so and turned Henry Pym, scientist who could shrink himself into something new: Superhero Ant Man, alias Henry Pym. This book was always very hard for me to locate. I owned maybe eight copies in all my days as a comic dealer.
Third has to be The Incredible Hulk #1. Now, this book was a Kirby brainchild all the way. And it had some really dynamic covers. Especially that first issue. But let me tell you--it was always among the very toughest of issues for me! Only a few times did that book ever cross the threshold of my shops. I found about the same number of these as I did of the first appearance of Ant Man, but generally in lesser condition for some reason.
Fourth toughest would be Tales of Suspense #39, the first appearance of Iron Man. Another Jack Kirby creation, Kirby brainstormed the character, did the layouts and rough dialogue for the book, then handed the project over to Don Heck who tightened up the pencils and turned in the final project. Because this book was one of the issues where Marvel introduced a new superhero within the confines of their old monster title, it was always one of the tougher ones for me to find. I've owned more than a dozen of them over the years.
Next would be Journey Into Mystery #83, the first appearance of The Mighty Thor, another concoction of Jack Kirby's. At this point Kirby and Ditko were being given the chance to produce new superhero books within the numbering of the science fiction and horror comics they were already writing and drawing for Marvel. Sales of Journey Into Mystery must have been good, because this was a book that was never particularly hard for me to find. I've owned somewhere between two dozen and three dozen copies over my career as a dealer.
The following superhero books were always relatively easy for me to find. That is, I owned dozens of copies of each issue of the rest of these books over the years. Which is hard to believe, considering how much some of these books sell for.
Amazing Fantasy #15 introduced The Amazing Spider-Man. I've owned a LOT of these in my day. So many that I've lost count. It was rare that I didn't have at least one copy in stock. The same goes for The Amazing Spider-Man #1. There were times when I owned as many as five copies at a time. They were not--in the grand scheme of things--rare. They were always in high demand, of course, but not what I would term "rare".
And X-Men#1 was another book that was easy to find, but also very damned easy to sell. This was because the later X-Men incarnation was so damned popular--it made those early issues must-have for collectors with deep pockets. Tales to Astonish #35 was another one that was hard for me to sell when I landed them, because no one considered it the first appearance of Ant Man--it was just his first appearance "in costume" and therefore was a hard sell in the early days of collecting. These are all books that I probably bought and sold a hundred times each (or more) over my days as a comic book dealer.