Well, I don't know about Turi here, but I went for it in this order (and believe me this is the order of my preference):
1] The Godfather.
2] (Guess what Turi), The Sicilian.
3] The Last Don.
4] Omerta
I really didn't like omerta that much and would have prefered not to have read it at all as it tarnishes the picture I had about
Mario Puzo (but someone once posted something that it wasn't Puzo who completed the novel Omerta....I have no idea if this is true.)