We have sailed on 36 cruises on all the popular priced lines (most on Princess-16) as well as the Seven Seas Mariner (now Regent).
The best buffet in my opinion was Carnival. The multiple area by type of food as opposed to the one=-stop all food area means less congestion.
Best overall food was the Mariner but you are paying premium prices for it. Also, food was like that served in better restaurants on shore-excellent but very small portions.
Best general food overall was on Princess. This surprised me since years ago Celebrity was the winner but if our last two cruises with them is any indication, their quality has declined.