We had a cabin on Deck One once, and it was by far the quietest location of any of our four cruises. Not that the other locations were noisy but Deck one has only 42 guest cabins, compared to 246 cabins on deck 6. And, in my experience most the noise you hear in your cabin comes from the hallway, deck one is the best for quite.
Another plus is you never have to wait for the second elevator, since it always (almost) arrives at your floor empty.