Pretty much what is in the title. There is this list, and I see here and there events being described as "one of the closest call to nuclear war", but I wondered if there was the ultimate close call that was by far the closest we have ever been.
There isn't any way to objectively measure a "close call," but most historians tend to put the CMC as the "closest." Why? Because:
Both nations were actually very close to coming to actual blows (the US was seriously contemplating invading Cuba, which would have uncorked a huge powder-keg)
There were severe misunderstandings on at least the US side (e.g. the US did not realize the Soviets had already had operational nuclear weapons on Cuba, including dozens of tactical nuclear weapons that would have definitely been used to repel said invasion)
The command and control systems of both sides were not that good, meaning that it was entirely plausible for low-level commanders to instigate nuclear war, leading to massive escalation even if the heads of state did not desire it
The survivability of Soviet strategic nuclear was not at all guaranteed against the US forces, leading to "use it or lose it" situations (which could incentivize the Soviets "going first" or at least having a very itchy trigger finger)
The high level of credible tension meant that the cost of any mishap, accident, or miscommunication was very high, since it would be easy to interpret these as plausible evidence of the beginning of hostilities (and there were accidents, mishaps, miscommunications, etc.)
The entire Crisis was coming on the heels of other crises (e.g. Berlin) and part of a general attitude of crises that had led many people to believe a nuclear war was very likely under any circumstances
...and probably a few more that I am not remembering exactly. The only thing that even remotely parallels this is Able Archer 83 and that was far better in some respects than the CMC (e.g., command and control was a lot better by 1983; the Soviets were in a much more favorable position on strategic survivability and credibility than they were in the 1960s; the US was not actually contemplating actual war in 1983, etc.).
Again, these things are qualitative and different historians read the plausibility of escalation differently. But these are the kinds of considerations that are used when "ranking" these kinds of "close calls."
The others are generally a lot lower than these two. For many of the other "close calls" you don't have as many avenues to plausible nuclear exchange as you do with these. It is also noteworthy that all of the participants in the CMC believed it was a "close call," which is highly unusual.