"Best Cruise Line" is really which one that's best for you. I believe on looking at the luxury lines in terms of your personal interest and lifestyle. For example, Crystal has larger ships with lots of entertainment and on-board programs. But on the smaller ships you have more passenger camraderie and better itineraries.
The companies I list are all in the luxury category, with top food and service:
Crystal Cruises (980 passenger ships) -- two dinner seatings, best nightlife, large cabins. More days at sea and extensive on-board lecture programs. Best of the Crystal ships is Crystal Serenity which has more alternative dining. Passengers are very dressy. So-so itineraries. Free soft drinks, but alcohol and tips are extra.
Silversea Cruises (200 and 300 passenger ships) -- elegant and quite dressy, more intimate due to smaller number of passengers; excellent itinteraries. All-inclusive price.
Seabourn Cruises: (200 passengers) -- in my opinion, the best itineraries. Tiny yacht-like ships with little on-board entertainment or activities, the focus in gourmet dining. Several formal nights, the rest country club casual. No balconies, but some cabins have "french windows". All-inclusive price. The best suites: Silver Suites on the Silver Shadow and Silver Whisper.
Radisson Seven Seas Cruises (300 and 700-passenger ships) -- Wine with dinner and tips included; good on-board programs and spacious balcony cabins. Not as pretentious as Crystal and Silversea, more friendly.
I hope this helps. I've been on them all, and this is my take on the differences.