Schumacher. Raw speed, there are a ton of drivers that are almost on the same level as each other, but Schumacher brings the complete package into view. Nobody else maintained their drive for so long. He built Ferrari around himself and gave himself every competitive edge he possibly could.
I know, I know, most of you are going to say he was an asshole behind the wheel, but those very same people are the ones who will say Senna was among the best. It makes no sense, but anyway.