Carnival - not good for adults. Too many children underfoot and very noisy gambling area right on deck.
Royal Caribbean - great trip. Had inside cabin but it faced down into center mall so had floor-to-ceiling window into mall area so didn't feel confined.
Celebrity - Panama Canal trip. Great line, wonderful service and food. Definitely adult oriented, especially seniors. Lots of cabins for those with wheelchairs, etc. Possibly boring for younger adults.
MSC - Wouldn't recommend. Everything 5 languages, which gets old on trips out of Florida where almost everyone speaks English. Food was mostly good, but focused on Italian (lots of garlic). Service was spotty. Workers were mainly Indonesian. Didn't speak much English but worked their tushes off (checked and cleaned our cabin twice a day). Head bees were mostly Italian and specialized in walking around the ship in their spiffy white uniforms. Head bees also ran the services sector for cruisers. You had to line up to book tours. Often saw dozens of people lined up only to be told there weren't enough openings, or even saw the booking agent close the window on them just because it was his quitting time. If you wanted a cocktail during the day you had to search the ship to find which bar was open, etc.
Would recommend getting a balcony if you can afford it. Expands the size of a small cabin and is very peaceful.