Actually, performance-wise and NOT MOVIE-PLOT-WISE, I'd say that I humbly believe that Deniro was at his best in:

1: Mean Streets
2: Raging Bull, and
3: The Gang That Couldn't Shoot Straight...even though I only saw like 30 minutes of the movie, but I thought Deniro was awsome...

"..Your youngest and strongest will fall by the sword.."

"...now you gotta speak more than one language to pull a heist..." Pudge Nichols

"...Never shall innocent blood be shed; yet the blood of the wicked shall flow like a river. The THREE shall spread their blackened wings and be the vengeaful striking hammer of God..."