I would say Roy Thomas since he did a lot for Marvel during the Bronze Age of Comics with him creating and co-creating characters like Wolverine, Ghost Rider, Iron Fist, and even Luke Cage. Can’t forget his work on Conan either!
But I’m gonna go with Dennis O'Neil. His work on Hard Traveling Heroes (alongside Neal Adams) is why I’m mentioning him. The stories tackled social issues like racism, drugs, overpopulation, urban decay, and protecting the environment. That wasn’t something you were seeing in superhero comics at that time. While it may be dated, you can’t ignore the fact that a lot of the issues covered are still issues that we still struggle with in modern day America.
Though I will say the story about Native Americans felt awkward and that would probably be labeled as racist these days, even though it is trying to address the injustices that Native Americans experience.