Seriously, this is about umpteenth such thread I've seen started on this same subject in my short time here. I'd like to address the topic at length...
If you're reading this and have unlimited income and want the best of the best in food and service, real pampering. I mean nearly a crew member to every passenger to serve your every whim. If you want the cruise all inclusive or very close to it, below are your choices (in no particular order).
5. SeaDream Yacht Club
If all the above sounds very good but your budget is not quite unlimited but close. You can get close but not the best out there with the following choices (again, in no particular order).
5. Discovery World Cruises
The next category is sometimes hard to distinguish from the one to follow it. The reason is this list is mainstream cruiselines, but with a small exception (an asterisk if you'd like). There are two of the mainstream lines still considered "Premium" lines. Some feel they deserve the title, some do not. It can vary from ship to ship and sailing to sailing but as a whole are known for among the best food/service among mainstream lines (again, in no particular order).
2. Holland America
The next category is mainstream lines. Most of the time the value leaders in the industry. Where the masses go to have a great time on a reasonable budget. (again, in no particular order).
2. Royal Caribbean
5. Disney (not so much the reasonable budget part though)
6. Costa (European line)
7. MSC (European line)
There is another subset of cruises that fit the niche market. Either river cruises or true sailing vessels or some other carachteristic that keeps them a little out of the mainstream.
2. Peter Deilmann
6. America West (Majestic)
7. Cruise West
8. Star Clippers
9. Viking River Cruises
There are certainly other lines I've neglected to mention. The point of this exercize is that noone I know has cruised every line on the above lists. The question leaves so much to personal speculation. Not to mention that there are no details about age, lifestyle, activity level or any other qualifiers that determine what might be the best line for a specific person, couple or family. All the lines have strengths and weeknesses, even the ones at the top. If you're going in order to be anonymous and party, disco and be entertained every minute, those lines would be a big mistake. If you want to get away from kids there are choices above. If you want family friendly there are great choices above. One choice is not perfect for everyone who visits these sites looking for the easy answer to a complicated question.
I guess bottom line is we all have opinions and preferences. Sharing them is great and very helpful to people wanting input. There is usually more information needed to find out what the perfect fit is for the person seeking the advice.