It's inevitable that this post will go up. Is Schu the greatest ever? Personally, I think he is. He is not only a great racer. His transformation of Ferrari is a good study case for business schools.
Sure, he's used some unsavoury tactics, but so has Senna and other greats. I think if he wins an 8th title (and that is a big if), his records may never be broken. I don't see anyone ever winning 9 WDC although the drivers are getting younger and younger.
I also think Jackie Stewart deserves consideration. His calls to improve safety in F1 helped the sport through its dark, extremely deadly period in the 1970s. Racing would not be what it is today with Jackie.