Myself I would choose either of the Carnival cruises. We happen to like Carnival over Royal. But I don't think you can go wrong on any of the 3.
To help decide you need to see what the costs for each one is and decided then, make sure you are comparing the same type of cabin.
Star Princess 2005, Sun Princess 2005
Caribbean Princess 2006, MSC Lirica 2006 , NCL Pearl 2007, Majesty of the Seas 2008, Carnival Destiny 2008, MSC Lirica 2009, Carnival Valor 2009,Carnival Legend 2010, Carnival Liberty 2010,Carnival Fantsay 2011,Carnival Valor Feb 2012, MSC Poesia Dec 2012, MSC Poesia April 2013, Carnival Legend Nov 2013, MSC DIVINA Jan 2014, MSC Divina Oct 2014, Carnival Liberty Jan 2015, MSC Divina Apr 2015, Carnival Ecstasy Oct 2015, NCL Star Jan 2016