what about 'Thirteen Days' on the Cuban missile Crisis ?

Here are a few I'd add:
The Constant Gardener – brilliantly directed, beautifully filmed and acted John le Carre story about love, Africa and alive conscience
Cup Final – the Israeli- Palestinian conflict through the eyes of a small group of soldiers from both sides thrown together
Long Walk Home - wonderfully realized film on a seminal civil rights struggle, the Birmingham bus boycott
Matewan and Norma Rae – unusually powerful labor rights sagas
Missing – Jack Lemon excels as a father searching for the truth about his son’s disappearance following the overthrow of the Allende regime in Chile
Romero – Raul Julia stars as a conservative cleric who becomes a hero in the fight against repression in El Salvador
A Force More Powerful -- Stories from every continent of everyday people using nonviolence to fight dictators and injustice

I think 'The Wind That Shakes the Barley', a film about the British occupation of Ireland in the late 1920's, could be in there. I don't think it's very well known though.

Z is long before my time, but I just looked it up and it looks great, I will watch it soon, thanks for everyone pointing it out.

Another good Spanish Civil War movie is "Libertarias" , ostensibly about "The Free Womeen of Spain", and how they teach non-radicalised women about economy, politics, and invite them to join their militia. Depressing ending as with all Spanish Civil war films. Some brilliant action scenes and constant humour as well though. Come to that: Pan's Labryinth could be on here.

I must shamefully admit that the only one of them I've seen is In The Loop, but thats the best political film I've seen.
Also a big fan of The Deal, The Ghost and Frost/Nixon, all very recent.

Surely 'Citizen Kane' should be the numero uno in the above list.
The nexus of big business and media ( including its new creature the internet and its offspring ) was made abundantly clear. Xanudu represents the present consumer society writ large, with celebrity culture not too high profile in train. Exposes as aids to political assassination and the publication of intimate secrets are there in black and white. Thirteen Days, Fail Safe and Dr Strangelove and other such movies must always re-assure the American public that the US Defense Forces are top dog. It takes three sams to down the U2: a USAF bomber obliterates New York on U S President's orders; a lumbering B52 piloted by good old-boy Slim Pickens penetrates missile and interceptor defenses designed for Hustler/Valkyrie supersonic aircraft fleets.
Gullible

Citizen Kane is too contrived. Its a pastiche.
The Third Man is a lot lot better.
But I agree that Dr Strangelove must be mentioned. And Reds.

Best political film: "Burn" (alternative title: "Quemada"). Stars Brando/directed by Pontecorvo/terrific music by Morricone)

The Battle of Algiers is brilliant, and would be my number 1.
You should have included at least one film by Francesco Rosi, eg Hands Over The City, about political corruption in post-war Naples, with a spectacular building collapse, or Salvatore Giuliano, about Sicily, or The Mattei Affair. I remember Alex Cox once describing the latter to me as "the Italian Citizen Kane". He wanted to show it on Moviedrome but the BBC wouldn't let him. I should add that I don't know Alex Cox; I just bumped him into one day and we got talking about Rosi and what he said stuck in my mind. Italian directors used to make fantastic political films.

I would like to submit a few of my favourites:
Fog of War: 11 Lessons from the Life of Robert McNamara
Thirteen Days
Bob Roberts

To follow up the previous comments, I'd add Investigation of a Citizen Above Suspicion, Todo Modo or The Working Class goes to Paradise by Elio Petri. Political satire at its best, savage analyses of Italy's venal governing classes and the blackest comedy on the theme of Italy's "strategy of tension". Indeed this period produced much of Italy's greatest political cinema, which inspired the promising Paolo Sorrentino (see The Consequences of Love). A necessary mention also for Roberto Rossellini for Rome: Open City, and Germany: Year Zero - absolutely heart-wrenching humanism. Mmm what else...

Good list. But, like Balaraman (30 April at 14:31), I feel that Costa Gavras' Z deserved a place. His State of Siege,too, is worthy of consideration.

Citizen Kane was mentioned; I'd rather vote for Touch of Evil (-58): Political on all levels. How about Tarkovski's Stalker (-79)? It was the look ahead which Stalker offered to us...

I have to commend every single comment above because of two glaring omissions that qualify two of the most propagandistic films ever made (1) to justify the Iraq invasion ("The Hurt Locker" which demonises the Iraqis for daring to defend their ancient homeland) and (2)the outrageous Israeli "Walz With Bashir" which reverts to the same old boring victims in the holocaust justification for slaughtering the people you are occupying.
"Walz With Bashir" was one of ten Hollywood propagandistic movies released to coincide exactly at the time of the Gazan massacre to extract as much sympathy for Israel as possible at the very moment they were murdering Palestinian babies with white phosphorus.Anyone who argues against the fact that Jews own Hollywood has to have his head examined. I avoid seeing Hollywood trash as a result ever since.

Robin and Marian is a fine piece of Vietnam related work.