Jason Owen of www.definition.org says that John F. Kennedy was the third smartest President of the United States. He is ranked behind Thomas Jefferson (2nd) and John Quincy Adams (1st).
Mr. Owen writes...
"JFK was many things--including the third smartest president to ever hold office."
Barack Obama and Donald Trump were not included, Mr. Owen says, because there is no credible evidence available now as to their IQs*.
The smartest POTUS, John Quincy Adams, had the highest IQ of all the Presidents at 173. Jefferson came in second at 160 and JFK followed with 158.
Close behind are Bill Clinton (156), Jimmy Carter (153), and Woodrow Wilson (152).
The POTUS with the lowest IQ are Andrew Johnson (125.7), George W. Bush (125), Warren Harding (124), James Monroe (124) and U.S. Grant (120).
*IQ (Intelligence Quotient) is derived from several standardized tests which assess human intelligence. The term IQ was the brainchild of psychologist William Stern.
"Which Presidents Have the Highest IQs?", by Jason Owen, NEWS, January 21, 2016, www.definition.org/