LL Cool J definitely has a strong case. Other candidates I would argue are Ice Cube, who pretty much birthed an entire coast as a teenager. As much as LL was the man, Ice Cube changed the trajectory of black music, and more specifically Hip Hop. And you could also argue Will Smith/The Fresh Prince. He helped take Hip Hop to MTV, and sold a ton of albums. Where as LL was the ladies man, and Cube was the bad ass teenager, The Fresh Prince was the one who took Hip Hop international with his hit TV show and eventually a movie career bigger than both LL, or Cube.
This isnt slighting any of the greats who came before or after. It just that these three have careers that have all almost ran parallel to each other's, and I think they are equally important to the timeline of Hip Hop.