Albert Einsten
He isn't completely responsible for E = mc2 -- at least not in the way you think he is.
The most important part of the equation -- the suggestion of an equivalency between mass and energy -- had been proposed by a number of scientists including Friedrich Hasenöhrl, Henri Poincaré, and Oliver Heaviside years, even decades, before Einstein published his theory in 1905. Even the equation itself, in a slightly different version, had been published more than once before Einstein, who was indeed able to simplify the equation and put it into the form that made it famous