I've read The Last Don, Omerta, The Fourth K and The Sicilian. The Sicilian was my favorite by far. I would rank this thus:

The Sicilian
The Last Don
Omerta
The Fourth K

Oddly enough, I haven't read The Godfather. I'm hesitant, because I was drawn to Puzo by the first 2 Godfather movies, and it doesn't seem worth it, since the movies were so good.

Is this delusional thinking?

Just when I get out, they pull me back in.

Michael Corleone in The Godfather III