I've cruised three times on Carnival, and twice I've stayed on the Riviera Deck. In my opinion, I truly believe that the Riviera Deck is the best deck to be on. It's very, very quiet, not to mention that you hardly feel any movement (especially if you're in midship).
On my last cruise on the Imagination, I booked a gty ocean view cabin, and ended up on the Upper Deck aft. Never again. The only time I'll book a cabin not on the Riviera Deck is when I book a cabin with a balcony; other than that I will always book on the Riviera Deck. All the cabins look alike no matter what deck you're on, unless you book a suite.