In 1957, before China had nuclear weapons, its leader, Chairman Mao, said the following horrifying quote about nuclear war:
"I’m not afraid of nuclear war. There are 2.7 billion people in the world; it doesn’t matter if some are killed. China has a population of 600 million; even if half of them are killed, there are still 300 million people left. I’m not afraid of anyone."
In 1967, China had tested nuclear weapons and ballistic missiles. To prove its systems worked in the face of Western doubts, it fired the only nuclear-armed ballistic missile in history to an unpopulated region within its own borders.
Given China's early gung-ho attitude towards nuclear combat, it developed a surprisingly responsible and calm force.
China has just 280 nuclear warheads, and none of them are mated to delivery systems. China flies bombers and sails submarines that it calls nuclear-capable, but none of them have ever actually flown with nuclear weapons.
China's nuclear doctrine forbids first strikes and centers around the idea that China would survive a nuclear strike, dig its bombs out of deep underground storage, and send a salvo of missiles back in days, months, or years.
This essentially nails the idea of "credible minimum deterrence." Everyone knows China has nuclear weapons, that they work, and nobody doubts China would use them if it first received a nuclear attack.
Also, China has spent a fraction of the money the US or Russia has spent on weapons while conforming with nonproliferation treaties.
China has continued to build up its missile, submarine, and bomber fleets, but all without the scrutiny afforded to nuclear systems.
Because China's nuclear warheads don't sit on missiles, if China attacked another country with ballistic missiles, the attacked country could be fairly sure the missiles were not nuclear armed, and resist returning fire with its own nuclear weapons.
China has more big cities than any other country and a stands to lose more than anyone in a nuclear exchange, but the incredible restraint shown by the Chinese earns them the top slot in this ranking.