The world has seen many successful revolutionaries who have led their people from subjection and brutality into liberation and freedom. Nelson Mandela did this for black South Africans with extraordinary elan, but it was not his greatest accomplishment.
And the world has seen a number of sound economic leaders who have taken deeply troubled post-colonial countries from bankruptcy and poverty and delivered them into stability and economic growth. Nelson Mandela also did this, turning South Africa into a flawed but stable economic power and creating half a decade of vital economic development. But this by itself did not make him exceptional.