Most overrated:

President John F. Kennedy, who initiated our involvement in Vietnam, thereby beginning the long-term decline of the United States, and, while spreading the rumor that Adlai Stevenson had recommended a policy of capitulation to the Soviets at the time of the Cuban missile crisis, had himself secretly adopted precisely that policy.

Most underrated:

Henry Clay, for the reasons stated by Lincoln in his eulogy. Without Clay there would have been no Missouri Compromise, and without that compromise the Union would have been dissolved.