Henry Fonda played “The President” in “Fail Safe,” which is a damn terrific movie.
Ronald Lacey as President Widmark in Buckaroo Banzai.
Tim Robbins also played the President in Austin Powers: The Spy Who Shagged Me.
I didn’t see the movie, but in the book wasn’t Stilson just a presidential candidate? Or am I thinking of a different charaqcter?
A check at IMDB shows that Sheen has played both John and Robert Kennedy and also played the President in Medusa’s Child and Family Attraction.
In the book and movie, Stillson (Martin Sheen) is a congressman with future plans for the presidency. John Smith (played by Christopher Walken) shakes hands with Stillson and has a psychic vision of a future President Stillson starting WW3. Only in this future vision (in the movie; the book was less detailed on this point) is Stillson/Sheen actually president.
As long as we’re just mentioning “President” there was “President Traveller” from In the Line of Fire.
In the American Empire Series (by Harry Turtledove) Upton Sinclair is President of the U.SA., (Robert E. Lee and Wade Hampton were presidents of the CSA)
Jack Nicholson as James Dalein Mars Attacks
[ul]
[li]Donald Moffat as President Bennett in Clear and Present Danger[/li][li]Eddie Albert in Dreamscape[/li][li]Joss Ackland in The Strange Case of the End of Civilization as We Know It[/li][li]Alan Alda in Canadian Bacon[/li][li]Loretta Swit as President Barbara Adams in Whoops Apocalypse (a personal favorite)[/li][/ul]
Ulysees S Grant- played by Kevin Kline in Wild Wild West. If my daughter makes me watch that blasted thing one…more…time…
Michael Douglas was my favorite, though. Great speech.
Who played president in 5th Element? I forget.
Dooku:
Uh, not quite. “Traveller” is the Secret Service’s codename for the President (whoever he is); it wasn’t his name.
And Daoloth, in Bob Roberts Bob Roberts was seeking, and won, a Senate seat from Pennsylvania, not the Presidency.
George Bush (listens to the screams of soup_du_jour)
I will vote for David Palmer (24) because he could take all the other guys mentioned.
"Bobby" - (Jack Warden) Being There
James Earl Jones as President Douglas Dillman in The Man
Peter Sellers as President Merkin Muffley in “Dr. Strangelove”
Also, check out http://www.webrary.org/rs/flbklists/presidents.html
Berzelius “Buzz” Windrip in Sinclair Lewis’ It Can’t Happen Here. Trashes the Constitution and installs a Fascist dictatorship. 
The greatest fictional president ever was rock star Max Frost, in 1968’s Wild in the Streets. Probably the only fictional president to gain office by dosing the D.C. water supply with LSD.
Prez, the first teenage President, in the DC comic Prez.
Tangentially, it’s occurred to me that the Oval Office must be the world’s most duplicated room. In addition to the real Oval Office, there’s the copy of the Carter Oval Office at the Carter Center here in Atlanta, the Johnson Oval Office at the LBJ Library in Austin, presumably others at the Nixon, Ford, Bush, Reagan and every-other-Presidential-Library;
and of course the mock-ups we’ve seen in all these movies and TV shows.
It was Nichelle Nichols, the actress who played Uhura on Star Trek. I forget where I read that, but there are hints in the story itself.
Ummmm…you know, he actually was president…