There might be a few unknown Vasilis/Stanislavs, and a couple of actions/inactions of leaders (e.g., Kennedy) that prevented a world-wide disaster.
Favouring nomination of Kennedy.
Curtis LeMay was Chief of Strategic Air Command:
During the Cuban Missile Crisis in 1962, LeMay clashed again with U.S. President John F. Kennedy and Defense Secretary McNamara, arguing that he should be allowed to bomb nuclear missile sites in Cuba.
He opposed the naval blockade and, after the end of the crisis, suggested that Cuba be invaded anyway, even after the Russians agreed to withdraw. LeMay called the peaceful resolution of the crisis "the greatest defeat in our history".
Unknown to the U.S., the Soviet field commanders in Cuba had been given authority to launch—the only time such authority was delegated by higher command. They had twenty nuclear warheads for medium-range R-12 ballistic missiles capable of reaching U.S. cities (including Washington) and nine tactical nuclear missiles. If Soviet officers had launched them, many millions of U.S. citizens would have been killed. The ensuing SAC retaliatory thermonuclear strike would have killed roughly one hundred million Soviet citizens, and brought nuclear winter to much of the Northern Hemisphere.
Kennedy refused LeMay's requests, however, and the naval blockade was successful.
Favouring nomination of Kennedy.
It seems unlikely that we would have been looking at a missile crisis in the first place if Nixon had been president, so it's not clear that Kennedy decreased existential risk on net.
Stanislav Petrov is a rather famous person (of course only on Lesswrong, not in the real world).
But there is another Russian who saved the world: Vasili Alexandrovich Arkhipov.
On this day in 1962, at the height of the Cuban Missile Crisis Vasili Arkhipov prevented the launch of a nuclear torpedo and thus a possible nuclear war.
It's strange that Petrov attracts much more attention than Arkhipov. E.g. googling "Stanislav Petrov" produces 101.000 results, "Vasili Arkhipov" only 9.040 results. By contrast searching for "Britney Spears" generates about 295.000.000 results. Sorta depressing.
Anyway, let this day be the Vasili Arkhipov Day.