P&O, RCI are 4 star lines Celebrity who are owned by RCI is a 5 star line. If food & service are your priority, Celebrity would be most peoples choice everytime. You will read & hear mixed things about every cruise line, so you can only make a judgment by reading all the reviews & make a decision from that.
I ditto Wilba's answer - 5* line with 5 course meals every night - a class above RCI and a leap above P&O (IMHO).
The whole tenure of the cruise is much better, classier, nicer, which ever way you want to describe it. As the boys say 5* not 4*. Food, surroundings and service all have a big edge over RCI and P & O. There are none of these annoying announcements over the PA inviting you to Bingo or the Art Auction just the midday one from the Captain stating your position and the outlook etc. If you can go for the Concierge Class and have that little bit of extra pampering. Celebrity is one of my favourite cruise lines and I’ve done a few…. Ida.
I have done Celebrity Century as well as RCI
RCI are more "fun" than Celebrity attracting a younger crowd
Celebrity was very classy though
We thought the food was the best we had in 8 cruises
Little touches like chilled towels when you get back to the ship on a hot day are great
In truth you will probably have little to grumble about with either of these.