To further complicate things, there can be differences between ships within the same cruise line. For instance, Norwegian Crown and Norwegian Sun are very different, as are RCCL's Splendour of the Seas and Radiance of the Seas. It's great to have so many choices, yet hard to decide between them all.
As to which line to try next -- what are the reasons that draw you to Royal Caribbean? They may be offered on other ships of other lines, as well. Knowing that can help us make some recommendations.
BTW, I've been on about 10 cruise lines, and it's hard to play favorites.