Many people compare the different cruise lines to hotels. If you were going to take a land vacation would you choose to stay at a:
Holiday Inn
Comfort Inn
Econo Lodge

Celebrity is know for excellent food, service, venues and the best specialty restaurants.

RCI has more on-board activities on some of their ships, all of the others are not quite as good as Celebrity. I have been told that they no longer serve lobster as a money saver, but I did not see them lower any of their prices! In our personal experience their service was terrible.

NCL can be very nice if you like free style dinning. Some love it and some hate it. I have not found many middle of the road on this - we have not tried it.

Princess is a nice cruise line - very port intensive - we have not tried this line yet as we really enjoy sea days and do not want to give up this feature.

I think that just about all of the Caribbean Islands and Mexican Ports have historical features to visit and I do not think you would have any difficulty finding things to do on any itinerary you would choose. We have not been anywhere that we did not enjoy - we especially love San Juan and St. Martin. Pick a place and a Cruise Line and go. Nothing is perfect, if there is a part of it that you did not like, change that part of it and go again - keep doing that until you get it right