Here's my choice: Josiah Willard Gibbs.
Years ago, as a graduate student in physics, I kept coming across Gibbs' name -- Gibbs' phase rule, Gibbs' paradox, Gibbs free energy, the Gibbs-Helmoltz equation, Gibbs functions, Gibbs ensembles, and so on. The name popped up in texts on chemistry, mathematics, theoretical mechanics, optics, and thermodynamics. Sometimes the latter subject seemed entirely a Gibbsian invention. Almost every branch of technology benefited from his work, especially the chemical industry. Alloys, explosives, fuels, and medicines were all touched by his genius.
Gibbs (1839-1903) lived all of his life in New Haven, Connecticut, a bachelor in his sister's house. He seldom traveled. Even his fellow professors at Yale University considered him something of a recluse.
As his work became known, universities in America and abroad bestowed upon him honorary degrees. He was recipient of the Rumford Medal of the American Academy of Sciences and the Copley Medal of London's Royal Society, the highest honor open to a scientist until the founding of the Nobel Prize. So modestly did Gibbs absorb these accolades that even his friends were unaware of the honors until they read of them in his obituary.
Gibbs' physics was the one great cornerstone of 19th century science that survived unscathed the relativity and quantum revolutions of the 20th century. Einstein and Max Planck, the architects of those revolutions, were late to discover Gibbs' work and were forced to reinvent many of the same results independently -- and with difficulty.
Toward the end of his life, Albert Einstein was asked who he considered the most powerful thinker he had ever met. He answered without hesitation, "Lorentz" -- referring to Hendrick A. Lorentz, the mathematical physicist, and then added -- "I never met Willard Gibbs; perhaps had I done so, I might have placed him beside Lorentz."