How about Appolonia's topless scene?
Just kidding.
From GF:
The sitdown with the heads of the various Families
The deleted scene from the first film in which Michael offers to help Sonny, where we get the first glimpse of Michael's ability to be a Don and very much like Vito.
GFII:
The answer Roth gives to Michael re: "Who had Frank Pantangelli killed?"
The flashback sequence at the end
GFIII:
Michael's confession to Cardinal Lamberto
Michael and Kay having lunch where Michael explains how regretful he is to Kay.
Godfather Returns: Me packing the book away in a box and shoving it into a corner of my attic