Proliferation for whom?

So, returning to the great question: who should have nuclear arms?

Theoretically this question is impossible to answer in any manner involving ethics. So I would suppose we'd have to deal with this one in the real world.

The "Official" Five Nuclear Weapons States: Good luck convincing them to give them up. Now, there is certainly a case to be made for reducing the size of their arsenals (with the potential exception of China, which has a very limited nuclear deterrent with regards to its peer competitors of America and Russia), but the balance of power, however repugnant the concept seems to many Americans, does have to be maintained when dealing with something like nuclear arms. As former, present and likely future superpowers (respectively), Russia, the USA and China all have the strongest cases for maintaining their nuclear arsenals. So what of the two European countries? Britain probably has the weakest case for maintaining an independent nuclear deterrent, given the general affinity between its own security interests and that of the United States. France would have a similar interest to disarm if not for the chance that it will lead the EU in any sort of combined-defense scheme, since it is more centrally involved in the European project than the UK and has a more powerful air force and navy than Germany. Were the EU ever to truly form a military alliance, France would be the natural candidate to provide its nuclear deterrent.

India and Pakistan: Again, good luck getting them to disarm. Even if India and Pakistan were to disarm bilaterally, this would not settle the region's nuclear question. Many in the West are not entirely cognizant that China is, and has been, more of an issue for India than America. They have border disputes, and fought a war over them in the 1960s. This is mostly settled now, but China's rise and support for Indian rival Pakistan could prove a sticking point. Then there's the darker possibilities of resource competition for not just fossil fuels, but possibly water control (a good number of major river systems in South and East Asia originate from China). Pakistan needs a nuclear deterrent against an otherwise militarily superior India, and India needs a nuclear deterrent against China. The system is in place, and it will take a long time and a lot of effort and goodwill to undo it.

Israel: The first important thing to note about Israeli nuclear weapons is that they've not done well as a deterrent against conventional attacks. They likely had nuclear arms during 1967, and this did not convince Arab states their conventional efforts would be useless (they built up troops that the Israelis then destroyed in the Six Day War). In 1973, the Arabs attacked directly and it took an American bailout to relieve conventional Israeli forces to win the war. Nuclear weapons did not deter the Arabs, they still had to be defeated on the battlefield. Though one might argue Israeli nuclear arms have prevented a similar clash since the Yom Kippur War, this probably has more to do with Israeli conventional superiority and knowledge that the US would bail Israel out as much as the nuclear threat. They work as a precautionary deterrent against WMDs such as Syria's chemical weapons stockpile and the potential Iranian (or Egyptian, or Syrian, or in the 1980s, Iraqi) nuclear device.

Speaking of '73, it's important to note that part of the reason the US bailed out Israel during Nickel Grass wasn't out of some great love for the state of Israel (we're talking about Nixon and Kissinger here, the former was pretty much an anti-Semite and the latter opposed the state's creation in '48!) , but out of Cold War politics. If the US didn't give Israel the means to defeat the Arabs conventionally, Israel would have had to use nuclear arms, and given the Soviet involvement in the conflict... That could have meant WWIII. Just another case of how 3rd parties make effective nuclear deterrence difficult.