"Greatest" is hard to define - most championships, fastest laps, brilliance of career, fastest in the wet, etc. etc.
I think that there's a certain level at which judgments such as "best" end up being purely subjective. I personally think that Senna was the most brilliant, the fastest at any given moment, and there's evidence to support that position. But there's just as much evidence supporting Fangio, Schumacher, Prost and a host of others.
But what about the guys who drove the brutish CanAm cars? It's hard to argue with the results posted by Mark Donohue, a brilliant driver and engineer, in a strange, almost completely unlimited series that was faster than F1 at the time, possibly by a lot. And the endurance drivers merit mention as well (Derek Bell, for example).
I personally don't believe that anyone could coax a car into outperforming itself like Senna (witness his performance at one of the Monaco races where be blew everyone out of the water, including a brilliant Prost drive, with a qualification differential so extreme that it boggles the imagination, or his results early in his career in inferior cars).
So I don't think that "best" can be so blithely assigned. "Among the best" is a safer proposition.