I have cruised on RCI, HAL, Princess, NCL, Carnival and Celebrity. Some of those lines I do not consider anymore when booking future cruises after issues on prior cruises.
My favorite for a long cruise is Celebrity. Celebrity has modern elegant ships, excellent service in cabin and MDR, Great guest lectures (on one cruise we had a retired WWII British Commander that gave a series of lectures on the rise and fall of German U-boats during WWII, other time had lectures on great ships through history), no need for the extra charge dining on Celebrity the MDR food is very good. Celebrity also caters to demographic with few kids on the ship. It is an over all upscale cruise. I also like my loyalty benefits of laundry, pressing, priority boarding and free cocktail party every night
When I'm in the mood for a short fun cruise nothing beats Carnival. Carnival is just fun, no need to dress up, good comedy club, Piano bar, shows, friendly staff, good service in MDR and cabin. MDR food is ok to very good and the steakhouse is great. We can not forget the people watching on Carnival.