Ranking global priorities is a hard problem, but it's one that a lot of smart people have already put a lot of thought into. The Copenhagen Consensus (http://en.wikipedia.org/wiki/Copenhagen_consensus) has come up with what I think is a pretty good set of answers.
Here's one item that I'd add to the list, which Copenhagen didn't consider: autonomous vehicle research. Car accidents kill about 42,000 people per year in the US alone, and injure 2.9 million. Sufficiently advanced autonomous vehicles could prevent virtually all of these.
Then of course there are all the "Singularity" ideas: prevention of aging, Friendly AGI, consciousness uploading, and so forth, but most of these aren't going to be accelerated much just by throwing more money at them.