Most people say it's Donald Trump but here's other research
Barack Obama, 2013: 1 million
Barack Obama, 2009: 1.8 million (generally considered a record for people on the National Mall)
George W. Bush, 2005: 400,000
George W. Bush, 2001: 300,000
Bill Clinton, 1997: 250,000
Bill Clinton, 1993: 800,000
George H.W. Bush, 1989: 300,000
Ronald Reagan, 1985: 140,000 tickets sold, but record cold moved the swearing-in ceremony indoors
Ronald Reagan, 1981: 10,000, according to the New York Times. This was the first year the ceremony was performed on the west side of the Capitol.