I voted Pacino, but it was a HARD choice. Brando, I will always think of him as the sexiest male ever in his sweat-soaked T-shirt playing Stanley Kowalsky in "A streetcar named desire" (a real masterpiece where you cannot easily tell who is the best)....
De Niro is a legend, he got a tremendous amount of talent (I recently rewatched GF2 and realized he was acting in SICILIAN!!!!) and it's a shame he's playing so many stupid roles lately (even though he's such a great actor he manages to let you bear watching). I admit my favorite actor was Bob, in fact, for a long long time. But now my pick is Al Pacino. I happened to rewatch many of his most important movies lately (Carlito's way, Scarface, Donnie Brasco and the GF saga) and his outstanding talent impressed me to the point that if I ever were given the chance to meet a movie star I'd ask for him, just to say him: "Thank you, Al. I love you, all of us love you"