We've done Disney, Carnival, Royal, and NCL.
Love Disney...hate the cost. Although I have my eye on the Disney Dream.
Carnival...loved the food, hated the party atmosphere (4 day cruise) and the neon design. Too Vegas-y in my opinion.
Have my eye on Celebrity's newest ships.
Generally speaking you'll get better rates on NCL. If cost is your main driver, NCL may be what you're looking for. We loved Any Time Dining on NCL (Gem) however, to us, the ship was underwhelming. Not much for kids to do and the public spaces were cramped and limited. In my opinion, it was due to so much space taken up for the various restaurants. Not a grand looking ship. Design-wise, we prefer Royal - with either it's atrium-type designs or the promenade-type designs (depends on the ship). We did a very, very port intensive cruise on the Gem, so we had very little ship time which was fine and the Gem fit the bill for that type of cruise. If I had picked an itinerary that had little ports/lots of sea days, then I'd prefer Royal. Food-wise, I thought they were about equal in quality. I will say we loved the outdoor buffet area on the Gem. This summer we're cruising on Oasis and I fear that it will spoil me rotten and ruin other ships for me. The variety that it seems to have will be hard to duplicate elsewhere.