My first was a Norwegian, and I loved it. Afterwards, I went on a Carnival and hated it. After this, I went on a Norwegian and hated them - they've gone really down hill over the past few years, gotten dirtier and the staff was very rude. Plus the food was horrible. However, my next Carnival was awesome - they've improved a lot.
However, recently I was on a Costa Cruise (in Europe) and I have to say that it was one of the best ships I've ever been on.