Jack Kirby was the superhero creator. Between the 40's and the 70's, he created or helped design many classic superheroes, including the X-Men, the Hulk, Captain America, and the Fantastic Four. He was also responsible for starting up whole new genres, like romance comics. His dynamic style, cinematic techniques and exaggerated, foreshortened action poses rewrote the rules for comic book art, and influenced a myriad of comic book artists.
I love his sense of design; the weird machines, grotesque characters and just the texture of his drawings.
If you want to know more about Jack Kirby, I highly recommend Tom Scioli's biography Jack Kirby - The Epic Life of the King of Comics.
In the early days of Marvel Comics, Jack Kirby defined the house style, and other artists were encouraged to draw in a similar style, so there were many Kirby imitators around, but I can't think of any who were anywhere near as interesting as the original.