It would depend on the person. If it was a family with active kids between ages of 10 to 18. I would recommend Royal Caribbean either Freedom of the Seas, Liberty of the Seas, Oasis of the Seas, or Allure of the Seas.
If it was young to mid aged adults looking for fun and lots of activities. I would tell them to try one of Carnival's newer ships. Freedom, Conquest, Splendor or Magic. There is a fun party atmosphere on these ships. Also Carnival has more activities then Royal.
If it was an older couple looking to relax and a high level of service. But not to picky about food quality I would suggest a Celebrity ship. My experience with Celebrity was excellent service, great production shows, no organized day time activites and really bad food. The worst prime rib I have ever eaten was on the Infinity. How to you screw up the most fattest cut of beef. I have made many a prime rib, it's easy as heck.
Just be able ship you chose is sometimes is more important then the cruise line. Not all Royal ships have flow riders and ice rinks. Also the Royal Promenade on the ships I listed before gives the look of a shopping mall. If that is not for you then these ships are not what you are looking for. So research not only your cruise lines but the different ships for each itinerary that interests you. I hope that helps.:-D