Good ole Tom McMillen. This is the first player I thought of when I read the title of the thread. A lot of guys have a little bit of gray hair (i.e. Irving or Draymond Green), but not sure there's anyone that has ever had as much as McMillen.
Kevin Love has quite a few gray hairs. And Grayson Allen, who's only 28, honestly isn't that far behind Love. By the time Grayson retires, he might be able to give McMillen a run for his money.