Instapundit links to a fun discussion on best portrayals of a real president by an actor in a movie or TV miniseries. Some of the suggestions include

Anthony Hopkins as Richard Nixon in Nixon
Paul Giamatti as John Adams in John Adams
Bruce Greenwood as JFK in Thirteen Days

I am partial to William Devane as JFK in The Missiles of October
and Ralph Bellamy as FDR in Sunrise at Campobello and in the miniseries The Winds of War and War and Remembrance.

Any thoughts? The role must be major in the plot of the movie, not a brief appearance.