We go on a cruise every year and have cruised most of the cruise lines. I did some consulting work for Royal Caribbean also. The best cruise line depends mostly on what your situation is. If you are young, single and like to party it will be different than if you are married and are bringing your kids and even different yet if you are older, have no kids and don't like them around.
We do the Disney Cruise every year with our daughter. It is expensive but an experience that no other cruise line can offer. It is by far our favorite. We also like the Western Caribbean over the Eastern. Cozumel is really rebuilding after the big hurricane wiped it out a few years back. The service there on their excursions are top notch and they treat you like they are glad you are there. The Caymans is also very nice with some cool stuff. Didn't care for the eastern caribbean that much. St. Thomas and the other islands are very pushy. You can't escape their hair braiders and street vendors as they are everywhere.
If you really want to know what the best quality cruiseline are, this was how they ranked back around 2000 when I did a lot of work for them. I don't imagine they have changed much.
Top of the line was Princess. We did an Alaska cruise on Princes and nothing has ever come close to their ships and service. Norwegien Cruise Lines is also very nice. Celebrity is also the upper level Royal Caribbean line. Disney is also considered one of the best and ranks number one in entertainment every year. Disney does not have casinos though. I did look at some recent reveiew and Disney is ranked at the top and Princess and NCL are ranked rather low these days which surprising to me but it has been over 10 years since I cruised these.
Middle of the road is Royal Caribbean. They have very large ships and a lot to do on board. The food and service is what makes them rank in the middle. Now don't get me wrong, the food is good, but other lines have better.
I can't speak for today because I think Carnival changed their business plan but back in the 90's they were considered the value cruise line. You could get great deals and thus you got some real winners on their cruises. Nothing you don't deal with in normal life, but in more concentrated numbers. My friend just did a Carnival cruise with his family and had a great time. Now I did get some great pictures from the top deck of a Royal Caribbean ship of the nakid sunbathing deck of a Carnival Cruise ship.