Definitely Vito's revenge on Don Ciccio.

The way De Niro just stares at Ciccio as he's being introduced is fantastic. You can almost imagine what's going through his mind at the time. "There he is...Don Ciccio...at last I shall have my revenge..."

Though it wasn't included in the poll (or in the final film for that matter - it was only included in the "Godfather Saga" as an additional scene) I would love to have voted for the scene when Vito attacks Ciccio's retired soldier with a boat oar while shouting "Give up the boy Vito Andolini!"