Going back to the thought of Paul's post...without a doubt, my answer always used to be, Royal Caribbean, and, I do have great memories, but, in truth, it's been many years since I sailed that brand.[as I type this,an RCI commercial just came on...too funny] Nowadays, I would "probably" say, Celebrity, loving the Solstice class.
I truly believe that matching the line to ones needs and wants is key to someone having a great cruise...I fit the HAL criteria, as example, but, would choose not to sail the line again, unless in conjunction with Cruisemates...
Answering this question is easier for a cruiser, who only cruises one line. For a cruiser, who likes to sail on various ships & has a well rounded cruise history, the answer is not so straight forward. Having someone who needs a minute or two to anwser that question, is a good thing