I'd go with The Man in the White Suit

Or maybe Tunes of Glory. Alec Guiness was also great as Smiley in the BBC productions of Tinker, Tailor, Soldier, Spy and Smiley's People. Oh and Lawrence of Arabia.

Actually, he was great in just about everything. And I always thought that he looked like a different person in each role he played. Jonathan Yardley, reviewing Alec Guiness: The Authorized Biography, agrees.

Like millions of others, I was captivated by his range, his sympathy, his wit and -- this above all -- his ability to lose himself so completely in the characters he played that they became utterly real and discrete. Unlike almost all other movie stars, whose presence as stars, as themselves , is always on the screen, Guinness was "a man of a thousand faces," none of them his own.

For this he was occasionally criticized, as being a superb character actor but nothing more. Gielgud, who was his mentor and whom he revered, cut him to the quick when, as a young man, Guinness aspired to play Hamlet. "I can't think why you want to play big parts," Gielgud said. "Why don't you stick to those funny little men you do so well instead of trying to be important?" Guinness's ambition was "to escape from the confines of character acting into the amplitude of a major role," as eventually he did on stage and in the movies, but when he got them he brought the character actor's sensibility to them. It was his greatest strength.

Unfortunately, Yardley doesn't like the book, which he says "suffocate(s) Guinness.