That may sound like a contradiction but let me try to explain via my own racing heroes - Fangio, Clark, Andretti and Peterson.
To my mind Fangio is still the greatest driver since WW2. It's the totality of his achievements - The success in the long distance South American races - coming to Europe at 40 years old to race in F1 - winning the Championship in four different cars and living into retirement despite seeing over 30 of his fellow drivers die.
However I still rate Jim Clark as the best driver since WW2. His uncanny skill, his smoothness and the ability to go faster than everybody else in anything on 4 wheels - FJ, F2, F1, Sportscars, saloons, and Indy cars - even Ford trucks I don’t think he reached Fangio's greatness however , partially because we can only judge him in a Lotus in F1 and , sadly, because he died quite young.
The same applies to Andretti versus Peterson. Their JPS years showed Ronnie was faster and there were very few drivers with Ronnie’s blinding speed. However, Andretti is , to my mind, the greater driver because like Fangio he raced in so many different types of car. From Sprint cars on the lethal dirt tracks of Pennsylvanian , though NASCAR, Indy, F5000 , Canam and F1.
I hope that sort of explains why I think "greatest" and "best" are two slightly different things in racing drivers.