The best cruise line is the one you love. In the luxury end, there are differences in lifestyle. Luxury ships are under 1,000 people, such as Crystal, Radisson, Windstar, Silverseas and Seabourn. Crystal, however, is the only ship with assigned two-seating dining. The others have open seating so you dine when, and with whom, you like. So, if you want Crystal, realize that this is part of the experience.
Personally I like Windstar Cruises the best -- casual clothing, superb dining and service. They are tiny and laid-back, everyone meets everyone else. This suits my personal lifestyle.
Seabourn has the best itineraries I think. No balconies but many cabins have "French doors" which open to the elements. Outstanding dining, superb staff.
Silverseas is probably the "best" in terms of elegance and dining. My pick for the best suites afloat are the "Silver Suites" -- this is the best cruise experience in the world.
Radisson isn't quite as pretentious and their ships are lovely, with four restaurants. There are dressy nights but the crowd seems a bit more down to earth than I've found on Crystal, Silversea and Radisson.