Keyser Soze (Usual Suspects) would definitely be on my list.

Michael Corleone (The Godfather)
Baron Harkonnen (Dune)
Jax Teller (Sons of Anarchy)
Ender Wiggin (Ender's Game)
Patty Hughes (Damages)
Mike (The Moon is a Harsh Mistress)
Kylara Vatta (Vatta's War)

Dr. Stangelove (Peter Sellers, Dr. Strangelove: Or I How Learned to Stop Worrying and Love the Bomb)
Frank C. Dobbs (Humphrey Bogart, Treasure of the Sierra Madre)
Danny Ocean (Frank Sinatra, Oceans 11)
Sam Harmon (Dean Martin, Oceans 11)
Professor Hubert J. Farnsworth (Billy West, Futurama)
Joh Fredersen (Alfred Abel, Metropolis)
Big Brother (George Orwell, 1984)

LOL

Farnsworth would be awful at diplomacy.

"You need to put in moves, Professor."

"Ah, huh? Whaaat?"

Sherlock Holmes
Scarlett O'Hara
Juror #8 (Twelve Angry Men).
Elmer Gantry
Hannibal Lecter
Andrew Wyke
Miss Marple

I like Ender Wiggin a lot, abge.

Dang, Kaiser, I almost put Big Brother on my own list (before seeing yours).

Andy McGee (Firestarter)
Charles Xavier/Emma Frost (X-Men)
Data (Star Trek TNG)
Leonard Shelby (Memento)
Marc Remillard (Galactic Milieu)
Sookie Stackhouse (True Blood)
Yoda (Star Wars)

lol Leonard Shelby.

David Xanatos (Gargoyles)
Gauis Baltar (re imagined BSG)
O'Brien (1984)
Lucille Bluth (Arrested Development)
John Cavil/Number 1 (re imagined BSG)
Lelouch Lamperouge (Code Geass)
And of course: Lord Vetinari (Discworld)

Honourable mention to Ryan O'Reilly from Oz.

Sherlock Holmes
James Bond (he can talk the others into revealing their plans)
Hercules Poroit
Ender Wiggin
Simon Templar aka The Saint
Gandalf
Paul Atriedes

I have one more to add to the list - the Culture GCU Grey Area. He/It would beat everyone mentioned so far.

Artemis Fowl
Ender Wiggin
Batman
Montgomery Burns
Sherlock Holmes
Beans(from Ender's Game)
Jimmy Neutron
(there combined brainpower will literaly blow up the game)

Mr Bean (England)
Inspector Clouseu (France)
Colonel Klink (Germany)
Borat (Russia)
any ideas for Turkey, Austria or Italy?

Guido (Roberto Benigni's character in La Vita e Bella) for Italy.

Dr. Cal Lightman from Lie to Me would beat all of them.

Heino (Tiroler Traditional Musician) for Austria?

This gives me an idea for a game...

Pick youur favorite character and then you have to play *as* that character. You annnounce who you are when the game starts and only one person can be a given character (so only one Sherlock Holmes) and the first to declare the character gets to play the character.

The Dude from The Big Lebowski ("This aggression will not stand man")

Vizzini from The Princess Bride ("Never go in against a Sicilian when DEATH is on the line!")

General Melchett from Blackadder ("Would that be the plan to continue with total slaughter until everyone is dead except for Field Marshall Haig, Lady Haig, and their tortoise Alan?" "Good god man! Even you know it!")

Cardinal Ximenez of The Spanish Inqusition from Monty Python (His chief weapon is surprise, surprise and fear, fear and surprise. His two weapons are fear, surprise and ruthless efficiency...)

Jimmy James from NewsRadio (He is a cypher, wrapped in an enigma, smothered in secret sauce)

The Monarch from The Venture Bros. (For is there any greater diplomat than his namesake, the mighty monarch butterfly?! Yes, there probably is.)

Wednesday Addams from the Addams Family (The version from the movies)

Me
Your mom

The other 5 don't matter.

(anyone sick of my 'your mom' jokes, yet?)

@Draug

Dibs on MadMarx.

Actually no. Replace Wednesday Addams with Otto from A Fish Called Wanda. I think a Buddhist approach would really benefit him in Diplomacy...wait...what do you mean the central message of Buddhism isn't "every man for himself"?

@Draug & Ulytau: Dibs on Dr. Strangelove.

Obviously, the best guy to play diplomacy with would be Leonard Shelby from Memento. You could stab that guy however much you wanted and still be allies with him.

"Dr. Stangelove (Peter Sellers, Dr. Strangelove: Or I How Learned to Stop Worrying and Love the Bomb)
Frank C. Dobbs (Humphrey Bogart, Treasure of the Sierra Madre)
Danny Ocean (Frank Sinatra, Oceans 11)
Sam Harmon (Dean Martin, Oceans 11)
Professor Hubert J. Farnsworth (Billy West, Futurama)
Joh Fredersen (Alfred Abel, Metropolis)
Big Brother (George Orwell, 1984)"

After careful thought, it is my feeling that Professor Farnsworth would go into Civil Disorder due to falling asleep, to be taken over by Andrew Ryan (Bioshock) or President John Henry Eden (The Bethesda Fallout version, not our friendly neighborhood Diplomat) whoever comes first.

Leonard also would be fun for an in-character game. At the beginning of each phase, you could explain to all the powers that you have this condition, could they please send press. Then you base your decisions on that the press from just that phase and the current board alignment. Well, that and the self-made tattoo on your body: Don't Believe Turkey's Lies.

I'm surprised that no one's said Doctor Who yet. So, Here's my list:
The Doctor (Doctor Who)
Gandalf (The Lord of the Rings)
The Professor (X-men)
Artemis Fowl (Artemis Fowl)
Vizzini (The Princess Bride)
Granny Weatherwax (Discworld)
Rincewind (Discworld)

I would love to see a game involving Vizzini, Batman, and Holmes. I don't even care who else is on the board.

I'd like to see how some of GRRM's ASoIaF characters would play Diplomacy;
Petyr Baelish, Varys, Tyrion and Cersei Lannister, Walder Frey, Doren Martell, and Daenerys Targaryen