Depends on who is doing the ratings and what their criteria are. One person's 5 star rating is not necessarily anyone else's. Beware of attempts to categorize ships, hotels etc. unless the criteria used are clearly explained and they can be objectively verified. All too often these ratings are very subjective.
Celebrity, RCI and Princess are all in the same General category IMHO. Ships vary as does personal tastes and expectations. The Explorer does not compare to any Celebrity ships at this time. RCI caters to a fabulous family vacation as well as to couples, singles and even seniors. It really depends on what you are looking for in a ship, itinerary, cost, etc. Each line has its good and not-so good points.