This is a very subjective question. For many of us it depends on the situation. Some lines are better on certain itineraries than others for instance. Or the itinerary of XYZ cruise line may offer more time in a port that you like, so you book with that line.
On average, I like RCCL and Carnival about equally. However I am probably going to do more cruises with RCCL in the future, with the goal being to gain more points for the Crown And Anchor Society since they have some nice perks. Carnival's past passenger program isn't too good, but RCCL's is very good.