Jackie Stewart is the first to say he is not a natural driver. That is the only reason I don't rank him right at the top. He is amongst the best ever for sure, but not a natural. He had to work at being good. People like Fangio or Clarke could leap in any car and be competitive pretty much straight away.
But there is no question at all about him being a true gentleman. One of the last of that breed. It is a real shame that big money has hit the sport (most of it in Ecclestones pockets!). It has killed real motor racing.
One thing Jackie said was that Michael Schumacher makes more mistakes in one race that he made in his whole career. The difference is that a tiny mistake in his day would have you off the track and maybe even kill you. These days, all the safety measures (quite understandably) mean you can get away with minor errors. There were, of course, many deaths during his time in racing. That is one element we are better without.