In my view, the best country is the country you will actually go to in higher risk situations. Finding the geographically ideal country doesn't help much if it will take you a month to apply for a visa, or you can only stay as a visitor for a short time, or if travelling and staying there will effectively bankrupt you (so glad to see cost of living on there!).

Of course, after a nuclear attack happened, most of those concerns would no longer matter, but before a nuclear attack happened when you're travelling they'd be very important! And it's important for your plan to work both in the cases of nuclear war happening and nuclear war not happening.

Hi, I have quite a lot to say about this, but I'm actually currently writing a research paper on exactly this issue, and will write a full forum post/link-post once it's completed (ETA June-ish). However, a couple of key observations:

1. Cost of living is likely to be irrelevant in nuclear aftermath as global finance and economics is in tatters (the value of assets will jump around unpredictably, eg mansions less important than electric vehicles if global oil trade ceases), prices will change dramatically according to scarcity, eg food prices. 
2. Energy independence and food security are probably the most important (>50% combined index value) because without energy food production is slashed to pre-industrial yields, and without food security the risk of unrest is very high. 
3. Latitude and temperature are less important than the impact on specific countries, eg temperature change is important not mean temperature, tropical crops like rice will die in a single frost. Europe could suffer -20 C or -30 C temperature change according to climate models, which makes agriculture impossible. Yet Iceland with vast fish resources could potentially increase food production. 
4. Rainfall could have a massive impact. The tropical monsoons could be very disrupted and are essential for agriculture in many areas. 
5. The could very well be almost no trade taking place in a severe nuclear aftermath as nations struggle internally, or due to fuel shortages (many countries are dependent on oil for agriculture at scale). Without trade many countries are fragile in areas of energy and manufacturing. Many component parts of power generation facilities, electricity & food distribution and communications infrastructure are manufactured in only a few places and within a few months without imports/exports such infrastructure may fail (eg lubricants, spark plugs, transformers, fibre optics, etc). Expect most things to grind to a halt without trade. 

There is a lot more that could be said but you're right that the large South American food producers (Argentina etc) look relatively more promising, as well as the usual suspects NZ & Australia. Though each will have severe problems in an actual nuclear winter and organisation such as food/fuel rationing and distribution from rural to urban areas will be immensely problematic. Not to mention the need for public communication processes to ensure people know there is a plan and survival is possible, again to avoid societal mayhem. Social cohesion, and stability indicators are probably very important. 

One problem with composite indices is that very low scores on one dimension can be masked by reasonable scores on others. Countries should be ruled out if they fail on a critical dimension. 

Finally, the act of 'escaping to' the 'most promising' location is not generalisable, and so the ethics of it are questionable. As Kant notes, the test is 'what if everyone did the same as me, would that undermine the institution in question?' and in this case it seems like the answer is yes. 8 billion people fleeing to Argentina would defeat the purpose of acting ahead of war to maximise the chances of each particular country. Carrying capacity calculations are important here too. I haven't even considered HEMP yet, which could very much complicate matters. 

The following case study is particularly illuminating of the problems even 'good' locations like NZ might suffer: https://www.jstor.org/stable/4313623?refreqid=excelsior%3A166e17f569637767a9caded49a1ced42  contact me if you want the full text. 

Maybe I can give a few thoughts on Chile to help your search.

• Average temperature may be confusing, given that Arica is currently too hot for agriculture, and Tierra del Fuego is too cold for agriculture,  I think a temperature change would just change where the agriculture was happening (this seems really critical, changing Chile's temperature rating can make it overtake Uruguay in your spreadsheet)
• Chile already has desalination plants for the mining industry, which is promising for building more desalination plants to support agriculture (as I can't predict what effect a nuclear winter would have on rain patterns)
• [If this is about where to move to] The speed and transparency of the visa process is important, as a recent immigrant to Chile, I'd say transparency is good and speed is average (~3 months)
• [If this is about where to move to] Population density and sentiment towards immigrants seem important - outside of the Region Metropolitano, there is very obviously space in Chile - and it is a great place to be an immigrant if (and only if) you have a white collar job

Not specific to Chile

• Years of education is a component of HDI, if you have already completed your education, I don't think this effects you so much (also caring about years in education rather than quantity learnt seems to be missing the point). As such I'd recommend using GDP per capita (PPP) and life expectancy
• Crime is likely to spike in a crisis, I'd prefer to be somewhere with low current crime rates
• I think you should penalise landlocked countries, as this does add a dependency on other countries and you need things to go right in more places
• When I look at the top places in the Food Security Index, I have places like the UK in #3 spot, despite the fact that it imports a large share of it's food. Even if I rank by the "natural resources and resilience" sub-index - I see net food importers surprisingly high in the list (and Norway and Finland are #1 and 2 in the natural resources and resilience sub-index. I would not like to be farming in Norway and Finland after global temperatures drop)
• I am somewhat sceptical of the global peace index - just based on a quick investigation, it has the UK as more peaceful than Chile - scoring Chile worse in "ongoing conflict" and "safety and security" - this is despite the UK frequently deploys troops abroad, the UK has multiple regions that want to break away, and Chile has not been involved in a war since WW2 - I haven't fully unpacked this, but to me it doesn't pass a sniff test