Robert De Niro and Al Pacino are, arguably, the greatest film actors of their generation; perhaps only Streep and Nicholson enter the same sphere. Pacino and De Niro have been onscreen together four times, not counting documentaries. One of those films, 'Righteous Kill,' was, at best, a tepid artistic success. The other three, however, seem to have provided worthy vehicles for the two actors' caliber of talent and skill. Which of these three De Niro - Pacino collaborations is your favorite?