There’s a limited amount of pleasure to be had in the really early superhero comics (1938 onwards), but there was a real leap forwards in the early ’60s, thanks to two people at Marvel comics – and no, I am not including Stan Lee in that.
Jack Kirby
Kirby had been working in comics since the late ’30s, and had co-created Captain America back then. When Marvel heard that DC’s new superteam book, the Justice League of America, was selling well, they decided to create one. It was a slightly rough job, and not a lot of thought went into the early issues, but the Fantastic Four was a huge hit, and Kirby created lots more characters – the Hulk, the Avengers, the X-Men, and lots more, and he also revived Cap and others. He brought a power and excitement to superheroes, indeed to comics, that had never been seen before, and for several years he created most of what has sustained Marvel for nearly 50 years, and defined the style of superheroes ever since. I don’t know if there has ever been someone with as fertile an imagination, in any field, and to have sustained such standards for so many years, working at such a fast pace (much, much faster than any other modern Western artist), makes it even more extraordinary.
Steve Ditko
Marvel’s other leading artist at the time was a different kind of creator, with art more lithe and agile, and often slightly sinister. This suited Spider-Man very well, and this title defined much of the soapy approach that was new to superheroes, and continues today (and Stan Lee was certainly a contributor to this). He created lots of great villains and stories in his 40 issues on Spidey (including two annuals), plus another great series in Dr Strange, and some of the most memorable images and sequences ever in comics.
Stan Lee
First off, there is every reason to believe he created none of these characters, nor did he write the stories: he took the stories, already drawn and written, and changed the words on the page. This was an important task, since neither Ditko nor Kirby was a great wordsmith, but he also created a house style of matey, almost-hip talk that drew fans in and made them feel a part of Marvel in a way that they couldn’t at DC, their bigger and more established rival.
Marvel has produced lots of big, cheap, B&W reprints of their classic comics. The first five Fantastic Four volumes (#3 has the material most beloved by fans, including first appearances for Galactus, the Silver Surfer, the Inhumans and the Black Panther (the first black superhero)) and the first Avengers collection have much of the best of Kirby, but any of the volumes where he is the main artist are well worth having. The first two Spider-Man collections and the first Dr Strange have most of the best Ditko. Also well worth a look: Essential X-Men 1, all the Thors, the first couple of Captain Americas. My old overview of some Essentials.