For me, the best scene of "On the Waterfront" is when he's walking with the girl along a play ground. The actress Eva Marie Saint loses a glove. Marlon picks it up, plays with it and finally puts it on his own hand. I love these improvised elements. He did a similar thing with the cat in the Godfather's opening scene and the orange pieces in the Vito's death scene.

Anyway, I think Marlon delivered his best performances in The Godfather and A Streetcar.