100% tax on all income over $1b. It's administratively doable and has been functionally done before (94% on all income over $200k in 1944), we just lack the political will.
"There are more than 144 million US taxpayers, and the billionaire class numbers less than 750. Solutions to wealth inequality abound, but first, we must persuade a volunteer army of lower- and middle-class defenders to break rank."