Country level indicators of altruism
A traditional approach to studying altruism has been to do some kind of economic game. E.g. seeing how nice people are in ultimatum games. One such study:
Oosterbeek, H., Sloof, R., & Van De Kuilen, G. (2004). Cultural differences in ultimatum game experiments: Evidence from a meta-analysis. Experimental economics, 7(2), 171-188.
Levine, R. V., Norenzayan, A., & Philbrick, K. (2001). Cross-cultural differences in helping strangers. Journal of Cross-Cultural Psychology, 32(5), 543-560.
They tend to produce suspicious rankings:


Perhaps owning to terrible sampling, in terms of size and representativeness.
Objective, large data measures?
Surely, we can do better. How about proportion of GDP/GNI donated to foreign aid?

One probably wants to regress out country GDP from this, but this overcorrects to some degree because part of the reason why the altruistic ones donate so much is that they are also nicer to each other and this promotes country performance.
Friendliness to other-race immigrants (via Twitter):

I also suggested that one could measure pro-social behavior on online helping sites like StackExchange. Thousands of people take their time to answer others' questions (contributors/altruists), but most people only read answers already given (leeches/lurkers). So one could take users per capita and regression out traffic per capita. One could also analyze the top users which more directly measures the , á la the method in my top mental sports players study.
https://twitter.com/KirkegaardEmil/status/1011834056114360321
(True to my people, my altruism score is high according to this metric.)
I predict such derived metrics would correlate highly with the above two altruism metrics, at least > 0.50.