Probably Senna for shear ability, but like Schumacher (Also one of the all time greats) he had a ruthless, quite unpleasant streak that you wouldn't see in others of his equal, for instance, Jim Clark, Jackie Stewart, Juan Manual Fangio.
But there are so many others you really can't discount;
Gilles Villenueve, Ronnie Peterson, Jochen Rindt, Stirling Moss...... it goes on for ever, people like Nuvolari, if you want to go way back, or more up to date people like Prost and Mansell.
TBH I liked James Hunt, I liked his style, bare foot in the paddock and Girls in the hotel room and all that, but he wasn't on the same level as the others mentioned above for shear driving ability.
The argument will run and run as too was the greatest, it will never be answered though as all the drivers are or were products of their time.
The modern crop of drivers are probably no less talented, but the political climate tends to remove their character traits making them all seem boring.
Bit like the races really.......