My opinion is that the largest problem humanity faces... is a combination of many of the aforementioned. Things like, a lack of education, global warming (though, I think people tend to glorify this one a bit), over population, etc. However; I think that if a proper space program was created, one that wasn't afraid of ambitious goals (like the NASA of the sixties) many of the issues we are facing now, could be 'cured.' The problems associated with the manned exploration of space lend themselves to creative answers, which then (eventually) trickle down to the general populous in the form of useful products (like memory foam, scratch resistant lenses, water filters, telecommunications, etc). Recap 'Biggest' Problem = loads of smaller problems solution (My opinion) = Manned space exploration