For me, it's between The Third Man, and either a David Lean film (I'd pick something earlier like Brief Encounter or Great Expectations), or else a Powell/Pressburger film (The Red Shoes, A Matter of Life and Death, The Life and Death of Colonel Blimp, or Black Narcissus). Or I suppose I could pick an Ealing comedy like Kind Hearts and Coronets. Dammit, it's too hard to choose.