Leo Szilard
Physicist and Biophysicist of Hungarian origin. He was one of the ealiest to notice the possibility of the chain reaction of nuclear fission: the essential idea for making the atomic bomb. He is best known as the person who advised Einstein to write a letter to President Roosevelt (1939) urging to begin a project to make the atomic bomb. And toward the end of the Pacific War (after the defeat of Hitler's Germany), he made a campaign in order to prevent the use of the atomic bomb against Japan. He was also active in the Pugwash conferences for abolishing nuclear weapons. Thus he is usually known as the model of the "conscientious and responsible" scientist.
However, we have to be careful. Although we do not have to deny the sensitivity of Szilard to international affairs, there remain some obscurity about Szilard's motivations. Some colleague scientists (see Wigner's recollections, in particular; Eugene Wigner was one of Szilard's best friends) and some historians doubt the purity of his motivations; didn't he make such a campaign in order to leave the documents that there were indeed "conscientious scientists" (himself, in particular) who questioned the justifiability of the use of atomic bomb? It's the responsibility of historians to make clear this, and to ascertain the credibility of "his version of the facts." Look at the picture with Einstein (which is at least a half fake), which smells bad; Szilard has certainly succeeded in making himself a legend.
For the sake of fairness, see Leo Szilard Online too. See also Nuclear physics timeline; Szilard documents; The Responsibility of the Scientist