We have cruised Carnival, Royal Caribbean, Norwegian, Princess, and Celebrity over the years.
I like analogies...so I rank them like this...from a retail perspective...
Carnival = Wal-Mart
RCC = Target
Norwegian, Princess, and Celebrity are a mix of Macy's, Nordstrom, & etc...depending on the ship and itinerary.
Effectively...anything will be an upgrade over Carnival. 😉
We used Princess for an Alaskan cruise. We're using NCL for our upcoming Med cruise. Haven't used RCC since our kids graduated from the kids club age. And don't see ourselves on Carnival at any point in the future.