I'm not a billionaire (yet), but that's why 100% of my charitable donations go to the SENS foundation. Every marginal dollar has a much higher expected utility there than anywhere else that I've found.
Here is another place with high value per dollar:
Industrial Progress is a very important and practical cause, and this group has a very effective leader who is actually having some effect at persuading people (particularly people in industry) and providing them with better ideas. And it's cheap. SENS needs a billion. A few hundred thousand would be huge here.
Both SENS and CIP are hugely important ventures that people should look into in my book.