As for the actually most welcoming countries: most Muslim countries should definitely be at the very top of the list. It's a part of the religion, from what I understand. Everyone's really friendly and helpful to the guests, sometimes to the extreme. A couple of examples: 2 random women offered a friend of mine to house him for a couple of days (no, you perv, it has nothing to do with that), *while he was on a train* to Chechnya. A random guy from the other side of the street bought me an icecream, while I was in Iran. It was weird, but it genuinely shows how hospitable the average Iranian is. It is absolutely fucking ridiculous that they're at the bottom of that list.
I've also had a lot of moments like that in Georgia. For example, a random older guy just started talking to us on the street and showed us around the city for 2 days.
Generally, the less Westernized and the less wealthy a society is, the more hospitable the people around you are going to be. Also - the further you are from urban areas, the more hospitable people are going to be.
Otherwise, SUPERFICIALLY, it largely depends on how extrovert the people are in the given country. In Northern Europe people stick to themselves more, while in the South people are more talkative and trying to be more helpful (which is not necessarily the same thing as being helpful, as most people in Greece, Italy, etc. will just point to a random direction when asked, even if they have no idea where the place I'm asking for actually is located).
But, yeah, that's ignoring infrastructure, the number of Starbucks per square inch, foreign relations with the US, general safety and stuff like that.