[oblio]

My guess is that the best investments are in education and social welfare. Basically try to get people who are down up and keep people who are up from falling down during their worst moments (temporary loss of income, depression due to loss of loved ones, etc.).

Besides this, I guess the next best investment is in refining democracy and its safeguards, as well as the actual government, so that:

a) lobbying by corporations and very rich and powerful individuals doesn't corrupt the system

b) the system is as efficient as possible (there's always going to be inefficiencies but you'd want most of the tax money to actually be used for what they're meant instead of administrative overhead)

More than that, there's not much we can do. After all, at some point we have to rely on free will :)