I'd say the best thing for billionaires to spend their money on would be whatever they're interested in. I think philanthropy is probably a poor use of resource

I'd say the best thing for billionaires to spend their money on would be whatever they're interested in. I think philanthropy is probably a poor use of resources compared to starting businesses with a purpose other than just maximizing shareholder value. A business where the goal is to just break even is a business that can focus on maximizing its positive impact on the world. Realistically speaking, no human institution will remain true to its original purpose for very long after the death or departure of its founder. Institutions drift, whether towards maximizing shareholder value (if a publicly traded, i.e. de-facto ownerless, corporation) or towards pushing a political agenda (if a non-profit foundation). The only way to have a chance of avoiding this drift is for the founder to pass on true ownership of the institution to somebody whom he believes shares his vision. Even then, that doesn't exactly work out perfectly because eventually the owner is going to make a poor choice for his successor and the successor will either run the institution into the ground or sell the institution because he doesn't actually want it.