<human>:  What was the most important event of the presidency of Barack Obama?
<bot>: The most important event of his presidency was the Affordable Care Act. The Affordable Care Act was signed into law by President Barack Obama on March 23, 2010. It was a landmark piece of legislation that extended health insurance to millions of Americans. It was a huge accomplishment for the Obama administration, and it is considered one of the most significant pieces of legislation passed since the passage of Medicare and Medicaid in 1965.