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The Godfather trilogy
I won't dare say much about these movie except that it is truly rewarding if you "get it". I didn't "get it" on the first viewing, a bit on the second, but did on the third and it was truly amazing. The first is enough, and although many say that the second actually adds on and is better than the first, I must respectfully disagree. The second is good, but in my humble opinion, cannot be said to be better than the first. The third, however, would immediately elicit sneers from true fans. The third, well ... I can only say that it is a fitting ending to the saga.
One truly memorable bit from the trilogy: Can you get the Nina Rota tune out of your head ? I couldn't, and I loved it.