I know Beyonder is not exactly what we're looking for (cosmic entity) but still, he made a gigantic impact on the storylines of many heroes/villains.
And though he's not from Marvel (I think) my vote goes to Zorro, because he was the inspiration of countless rip offs (normal, somewhat boring looking guy who becomes a cunning and noble crimefighter wearing a mask and a special outfit, who has a hidden base somewhere - with a loyal servant - and whose identity is known only by very few people).
Without Zorro, no Batman, and without Batman, no Spider-Man.
Marvel heroes/villains who have had much influence : the Watcher, Hulk, Magneto and Kingpin (to name a few).
More down to Earth people : Jonah J. Jameson. Whenever a hero is upset and smashes things because he got a bad review, it's a safe bet the article was written in the Daily Bugle 😉.