A few of my faves:
"In my home! In my bedroom where my wife sleeps, where my children come to play with their toys, in my home."

"I know is was you Fredo, you broke my heart, you broke my heart."

Michael's and Carella's dialogue in GFII, in which he says at the end in Italian; "Times are changing."

"I spent my life trying not to be careless, women and children can be careless, but not men." and the entire garden scene.

Michael's and Tom's dialogues in GFII;
Tom: "I always wanted you to think of me as a broter."
Michael: "I know."

And also;

Tom: "I've always ben loyal to you, what is this?"
Michael: "You stay?"
Tom: "Yes, I stay."

And also countless of others.