De Niro. Absolutely. Pacino has cool factor, tho.
This is sorta like comparing Samuel L Jackson to Forest Whittaker. A personal preference as opposed to a true test of ability. A'course there is always the G_dfather..
But I digress. Their choices in movies define my answer. De Niro. Heat, Raging Bull.. but I liked Pacino in Devil's Advocate..among, geez.. too many to count on either side.
Peace.
(Dont nag me on the Sam-Forest comparison, Samuel can act when he wants, and Forest Whittaker can joke.. Mace Windu.. Forest in 'Ghost Dog'..gee.. I hope I can find a ******* with Ghost Dog, I wanna see that again.)