Hard to tell, because he did several important things, whether or not you agree with them.
I think his biggest achievement was keeping the American financial system from a complete collapse following the huge banking and real estate crisis of 2007 and 2008. And putting in place an economic stimulus program that helped the US economy recover -- too slowly, for many people -- from the potentially disastrous world recession of 2009.
Unfortunately, for many reasons including Republican resistance to his entire presidency, but probably also including his own ties to Wall Street, Obama's economic program was much more effective at rescuing the Wall Street banks than it was at helping bankrupt homeowners avoid foreclosures on their homes.
Similarly, his economic stimulus program was supposed to reduce unemployment below 10 percent within a year of passage; however, the problems of the economy were much deeper than most economists recognized at the time. Therefore the stimulus proved to be too small to slash unemployment rates immediately. The stimulus and the bank rescue efforts did work in the long run; what Obama did in 2009 helped to lay the foundation for steady US economic growth from 2010 through Trump's election in 2016,
But it didn't help enough low-income Americans -- white, black and "other" -- right away. That's probably one reason why Trump won in the Electoral College in 2010, although not the most important one.
-- democratic socialist