My favorite cruise line is, sadly, no longer in existence. It was Dolphin Cruise Line, which operated a small fleet of former liners in the 1980s and 1990s. The food was unbelievable --- there were midnight buffets every night that rivaled the grand buffet held on cruise ships today. Service was impeccable. Crew got to know your names. And, prices were affordable.
I don't have a favorite today, as I do find that most of the mainstream lines have a lot in common. However, among my favorites are Princess (for the service and variety of activities) and HAL (the S-class ships). I also have a fondness for NCL as it is the line we have been on the most -- we just have to adjust our expectations before we go, but we still manage to have a great time. (And, we'll be back in two weeks!)