After Lee / Kirby the best thing to ever happen to Marvel is Chris Claremont. Love his work or hate it, there is no denying that his work on the X-men defined Marvel from the late 70's to the mid 2000's as the X-men went from a single cancelled book to the franchise that dominated comics throughout the 80's and 90's. Even after he left X-men all Marvel and subsequent writers did was rehash his plots and cash in on characters like Wolverine that he made popular. You could argue that over the last 5-6 years Marvel has finally shifted the focus from the X-men to the Avengers but for the 25 years before that it was all X-men thanks to Claremont. Its why Marvel had such sales dominance throughout that time.