The already said quotes from Don Vito Corleone are also my favourites, expanded with the following:

I want to see what he's got under his fingernails.

The Italian quote of Young Vito in Sicily, when he finishes of one of Don Ciccio's goons. He repeats the exact same words the henchmen of Don Ciccio where saying 25 years earlier about the young Vito Andolini.

Hyman Roth's tirade to Micheal, about asking who killed Frankie.

Frank Pentangeli to Micheal saying "morte". I don't think he couldn't make it more clear what he wanted to happen with the Rosato bros.

And probably many many more...