I discovered Jack Kirby around 1972 (with KAMANDI) and quickly devoured his comics from the previous decade, especially Fantastic Four. Here’s a nice NYT article about 1960s comic books that sums up why they’re so great.
Just as Mr. Infantino’s Flash had captured the gung-ho futurism of the late 50’s, illustrators like Kirby and Mr. Ditko brought an appropriately darker, grittier and sometimes spaced-out look to Marvel’s pages in the mid-60’s. The Fantastic Four spent a lot of time on the Lower East Side, where Kirby had grown up. Mr. Ditko’s hallucinatory illustrations for the mystic Dr. Strange had a direct impact on the psychedelic art that bloomed a few years later. In “Electric Kool-Aid Acid Test,” Tom Wolfe reported that the LSD guru Ken Kesey spent hours studying Dr. Strange comics with acid-inspired probity. Rock-concert posters also lifted the character’s image.
Link (thanks, Scott!)