There is no "best" cabin. Everyone has a different preference. Do not book with the expectation of an upgrade. You may get it but most likely will not. And then it usually is only with in categories -- inside to inside, outside to outside, etc. Past guest has nothing to do with it. My friend, frequent past guest, recently book a inside guarrante and got a 4A directly under the casino on a sister ship to the Miracle. It was very noisy.
I prefer a cabin in the center of the ship, as low as possible with cabins above and below. Top deck of cabins receives noise from the lido deck above on almost all Carnival ships. Noise coming from below is not as bad as noise coming from above -- chairs being moved, walking etc.