I have read all books and rate them from best to worst;

Sicillian
Godfather
Fourth K
Last Don
Fortunate Pilgrim
Fools Die
Dark Arena
Omerta
The family

I would particularly recommend the much underated fourth k. It is a story about power and is relevant to a lot of the situations in the world today. Some of the writing is pure genius and I loved reading them out to my wife. I particularly like the sub stories just before the atom bomb goes of. They are very good and I feel Mario Puzo is just showing of how good a storyteller he is.

A man without sin is like a ship without a sail. The Oracle in the fourth K.