I am sailed 7 times in 10 years. I have sailed with Celebrity (a long time ago) and Royal (about 7 years ago).
All other cruises have been with Princess. I always thought that Princess was a bit better in every way.
While I do not know if other lines have improved or declined, I can say that Princess is declining. It seems like they are trying to save money and hoping we do not notice. Well, guess what-I do.
The cabin is a bit dated but still nice. The shower is very small. The steward was very good and we rewarded him for his excellent service. We were in the center of the ship and the ride was exceptionally smooth.