As stated, it depends on what you are looking for.
If I just want to get away and have fun, it's Carnival.
If I want to relax, be papered a bit and enjoy a nice mix of shipmates it's Royal Caribbean.
If I need an ego boost, and total pampering it's Celebrity.
If I want the worse possible trip, and something to scream about it's NCL.
With in each line there will be different sizes of ships, this will also effect the cruse. Smaller ships may not offer as many amenities, but the feel of the ship is much more warmer.
Another thing is no one likes to talk about is the cheaper the cruise, the more likely you are to travel with some 'Ugly Americans'. You can find these folks most anywhere, but I've noticed over the years they show their faces on the cheaper cruises more often. More than once I've wanted to get up and throw them overboard, or at least give them a good verbal lashing.
BTW if you are not familar with the term "Ugly American" it has nothing to do with looks, but personality.
As to my coment on NCL, they changed ownership a few years ago, and there service has dropped noticeably. I gave them 3 tries since the change, and will most likly NEVER use them again. Taking a NCL crusie is like riding a bus vs. an airplane, yes it will get you there, but...