Jackie Stewart would be my 3rd choice, but because of what many already said: turning a mediocre team into a winning outfit, to me defines the best driver. So, 2nd spot has to go to Lauda, and 1st to Schuhmacher, just for that.
Ferrari made some mistakes during his tenure, one was letting go of Lauda after the accident (and while I really liked Hunt, Lauda would have been a quadruple WC if not for that accident and missing some races).
Imagine even now if Lauda would be in charge at Ferrari. Would be a different team for sure.
Regarding talent, it's hard to say. When you look at the legends about Senna's wet speed (and Schuhmacher's as well) then it is more determination and relentlessness what then becomes talent. And relentless Senna was, indeed.
I remember the interview Jackie Stewart did with Senna after his crash with Prost. He was absolutely stubborn, even telling Stewart off.
And talent: Berger just to name one had huge talent, but he was soft in the end.
oh, and I always had a soft spot for Mansell. One of my favorite drivers with his special left right fool you style.