1. John Quincy Adams
Source: Biography.
Before being the 6th President of the United States (1825-1829), John Quincy Adams was a highly regarded lawyer graduating from Harvard University. He was also a brilliant statesman and fluent in at least four different languages.
Without even thinking about IQ and numbers, the man was smart, brilliant even. Some people decided to put a number to that brilliance, however, and they deducted that Quincy Adams had an IQ of 168, the bar none highest IQ of any president of the United States (so far).