Most overrated:

Henry Ford. Despite popular misconceptions, he did not invent the automobile, the assembly line, vertical integration, or mass ownership of cars. What he did was simply to hit the auto market at an early stage of development when all it wanted was an unchanging, basic black, cheap open car like the Model T. Having cleaned up on this stroke of luck, he then stood rooted in ignorance while automotive progress whizzed by him.

Most underrated:

Two nominees of many possible. Montgomery Meigs, quartermaster general of the Union armies. He created the military machine that Grant and Lincoln were able to use to produce victory. Or else Albert Gallatin, Jefferson’s brilliant Secretary of the Treasury (and much else besides). He is largely forgotten because he was disqualified by foreign birth from ever contending for the Presidency.