My vote goes to The Master And Margarita as it appears to have the only chance of coming in ahead of On the Beach. Not knocking that novel, but it was a lot more compelling back when it was written at the height of the Cold War. Now, not so much. That and I won't go into the overblown estimate of the world wide effects of nuclear fallout even from a hypothetical massive nuclear exchange between the USSR and the US at the time. For a more realistic account of the effects of fallout try the fictional but based on fact novel Black Rain by Masuji Ibuse about Japanese exposed to fallout from the bombing of Hiroshima.