Its really hard to compare line to line, each one is different, diverse offering something for everyone,kind of like comparing apples to oranges.
Making a broad-based comparison, IMO i would put Princess squarely in the middle, above NCL and just below Celebrity.
What we like about Celebrity is the up-scale clientele for a mass-market line, the food and service is quite good. My husband prefers set seatings, although the rest of us like some variety. Doing totally free-style though was not for us. The activities are less varied on Celebrity, and this year's itineraries to the Caribbean for 7 days are quite poor. We are 2 families traveling together with 2 teen-age boys.
Without a doubt for me - Princess first, Celebrity a close second and NCL (not for me cuz only have Free Style Dining and I like Traditional). Pluses for Princess - choice of dining - Traditional or Personal Choice where you can make your reservation each night; Celebrity Traditional only.
Princess fresh water pools (only cruise line that has them) - the rest are salt water.
Princess actual refridgerators which keep drinks and ice cold - Celebrity = mini bars (think, but not sure the same for NCL).
Food - equally good on Princess and Celebrity (been quite a few years since I took NCL) as is the dining room and cabin service.
I cruise the Emerald Princess, Eastern Caribbean on April 16, 2012