my favorite deleted scene (which was deleted, ironically, from the Additional Scenes of the Godfather Collection:

After Bonasarra leaves Vito's office during the wedding scene (opening scene), Vito observes his son Santino staring at the window. Vito is peeved at this, and whistles at him.

I only saw this little scene on the Godfather, A Novel for Television.

**
as far as what was shown, I think i loved the one where Tessio and Sonny are in the room discussing whether the traitor is Paulie or Clemenza, and Michael brings Tom's wife Theresa into the room with him. How Santino talks here is as good as anything else they showed in the finished film. ("where were you, where?")