The father of modern computers
Not so many years ago the name Alan Turing was obscure. Today Turing is finally gaining recognition for his notable achievements - as well as the ignominious treatment he received because of his homosexuality, which drove him to an early death in 1954 and led to a posthumous apology from the British Government in 2009. Ref. Source 4
Alan Turing was definitely one of the smartest men on the planet possibly ranking as high as the great Einstein although he was no where close to his popularity.