But in answer to your question - I would start a foundation with very smart and trustworthy people at the top, with the remit to determine the underlying causes of the most urgent problems affecting people - climate change, inequality, poverty, access to health care / education / justice / clean water / food etc, and the most effective way of lessening those problems.
Every billionaire out there could give 99% of their wealth to endeavours like this and still live very comfortable lives.
The fact that they don't speaks volumes about their character.