GF - Duvall and Pacino. Both of them show a large variety of emotions. For example, we see Pacino scared to death in the Restaurant scene. Never again we see such a performance of him again. In GF2 he's playing the "coldhearted bastard" very well who's getting furious twice: "Was it a boy" and when he's slapping Kay. Duvall's performance in GF1 is perfect. You never know whether you should love him or hate him. Best single performance: Duvall's "The Corleones would be outcasts..." Brando and Caan are brilliant too, but we see a narrower spectrum of emotion.
Here I have to admit something: I don't find Diane Keaton convincing.

GF2: Cazale. "I'm smaaht."
I don't think that Gazzo was half as good as Castellano. He must have been drunk all the time.

GF3: Talia Shire.
Here I also have to admit something: I find Sophia Coppolla convincing.