Answer:
The BEST" of anything, including cruise lines, is the one that suits you best. I've been on a few. What I can tell you is that each cruise line (and in some cases even the individual ships) have their own personality.
On one end, you have the mass marketed cruises, kind of analogous to an inexpensive subcompact import. On the other end, you have the Bently, Rolls, etc. The really funny part is the people who go cruising who know little or nothing about it. If you take a Carnival Cruise for x dollars and spend for the requisite liquor, spa treatments, shore excursions, etc. what will the final tab look like?
I actually did a comparison for a client between a VERY nice Premium Class ship in the Med. Vs. a Luxury line. Using average figures, the Luxury Line was actually a little over seven dollars a day more expensive on a ten day cruise going to the same places.
(Either will save you +/- $800.00 per person over a land based vacation to the same places)
How could this be? Simple, they don't charge for beer, wine, liquor, shore excursions, etc, AND they paid for round trip airfare from selected gateways in the States and put the clients up in a five star hotel the first night allowing weary travelers to acclimate BEFORE beginning their cruise.
And yet, if I presented both charges, to you, right now, if you didn't know better, you would most likely select the lowest "front money"...... Of course, that's not always the BEST VALUE. In short, it's not as simple as it looks. Professional advice and guidance are available for little of no charge. Find a Certified Travel Consultant, who guards his name and reputation, and avail yourself of her/his services. The time you invest will protect YOUR time, YOUR money, and YOUR ulcer.