Yeah, okay, in the comedy Dick Dan Hedaya isn't so much playing Richard Nixon as he is the "idea" of Richard Nixon, but his performance is so hilarious it doesn't matter.
Before he was President, Abraham Lincoln was a young lawyer struggling to establish himself. That early life is despicted in John Ford's Young Mr. Lincoln, where the future President is played by the incomparable Henry Fonda in a bit of pitch perfect casting.
The Cuban Misle Crisis was one of the defining events of that latter half of the 20th century, and Thirteen Days depicts the world inching towards nuclear war from the vantage point of the White House. As JFK Bruce Greenwood delivers a strong performance, making the character very human, rather than just an imitation of mannerisms and speech patterns.
Frank Langella rightfully earned an Oscar nomination for playing a post-Presidency Richard Nixon. In Frost/Nixon he creates a portrait of a man trying to rebuild his legacy while battling against his own worst enemy - himself. The performance isn't as funny as Dan Hedaya's in Dick or as sympathetic as Anthony Hopkis' in Nixon, but it is rivetting.
The second of two Presidents played by Anthony Hopkins in the 1990s (the first being Nixon). In Amistad he isn't the star, but his passionate performance as John Quincy Adams, which includes an epic court room speech, is one of his absolute best.