Outcault is widely credited with being the first creater of a strip having the same lead character. No regular hero's or villians without him. If you want to go back a bit further, there was the political cartoonist Thomas Nast, who gave us the modern image of Santa Clause, the Elephant for the GOP, and the Jack- for the (he was a , and that is not a donkey).
I am not sure I agree with placing Kirby over Adams. Adams quite literally changed the way artists set up their pages and drew their work. Compare, for example, Murphy Anderson to Adams. There really are few artists from the 1970's onward in mainstream comics whose work was not affected by him or his layouts, and his volume is actually pretty huge, by the way. Kirby captured a lot of hearts with his style, but I don't know if you can say his style was copied and used as a template for development.
Don't forget Miller. Comics are viewed by the media as a source of profitable movies and TV shows. Would comics, and their relatively low profit margins, still exist except as a niche product without The Dark Knight?