His work, circa ‘64-’66, was the best in comic history imho.
So many genuinely anthemic story themes and illustrative techniques were introduced during that run, not just in FF (which were epic) but in Thor, X-men, NF/Shield, ... literally anything Kirby touched during that time. Conversely, so little was actually produced once he left Marvel, that you have to wonder how much Stan Lee was really involved except in broad outline.
Post Kirby, not much was going on outside of Spiderman, which was not terribly innovative once Ditko and Lee had established the character.