each ship, even in the same cruise line has its own character and features that make comparison difficult. I told myself that i wouldnt compare my recent cruise on Carnival Triumph to the one before that on the RCCL Adventure of the seas. But I cant help it, both were big ships. the pools on carnival were smaller but they had a waterslide, RCCL had better entertainment, with an ice skating rink, rock wall, ect ect. the biggest difference i noticed was the level of service was much lower on our Carnival cruise. On RCCL I learned what a crumb squeegee was in thier dining room (pretty classy) on Carnival they danced on the serving stations (whoopee)..food was slightly better on RCCL with the dessert chef definately excelling. Buffet was often cold on Carnival. Dining room fare was repeated and friday seemed like leftover night on Carnival.
So I vote for RCCL because service and food are two of the biggest standards to compare. ship features, ports, and entertainment are too varied to compare.
We are going on RCCL Empress of the seas in Dec. and it is thier smallest ship, so compared to the last cruise with them, I am expecting a different experience from the same cruise line.
Bottom line is they are all good...my goal is to compare 6 or 7 cruise lines hopefully numerous times.