Orrin Knox from Advise and Consent
Kenneth Yamaoka from Eagle: The Making of an Asian-American President
Richard Wilkins, of course.
Mr Smith from MR SMITH GOES TO WASHINGTON
That’s like asking who are the most sympathetically portrayed puppy-kickers.
John Joseph Bonforte from Heinlein’s Double Star. (Spends little time on stage, gets practically no lines, but the story is really about his character, not that of the actor who doubles him.)
Josiah Edward “Jed” Bartlet for the win, of course! 
Vetinari.
Adam Selene. Had the good grace to die as soon as possible, for one thing.
Prez Rickard
Teen Girl President
Was he terribly effective though, always being distracted by clocks and Boss Smiley?
My nominations: Russell P. Kramer and Matt Douglas in My Fellow Americans. A Republican and a Democrat working together in the best interests of the United States? In this film, it happens.
Mayor Quimby
Another vote for Vetinari.
Zaphod Beeblebrox
Francis Urquhart. He’s the perfect politician - “ambitious, vengeful and manipulative man who lusts after power and is willing to commit murder and blackmail to get it”.
Vetinari, followed by Andrew Shepard.
You might think that; I couldn’t possibly comment.
Nevermind mine was taken.
While Vetinari and Jed Bartlett are contenders, the greatest fictional politician ever is obviously President Laura Roslin. Anyone who doesn’t agree with me can take a long walk out a short airlock.
Best: Jed Bartlett
Worst: Jake Featherston
Senator (and then President) David Palmer. Accept no substitute.