My answer.....Have you ever cruised before?
If no....Try Carnival
I always point a newbie towards Carnival because the cruises are cheaper and they have shorter cruises. Let's face it, a new cruiser might not like the experience and it's best to do a short cruise for less money then doing a longer more expensive cruise and then find out you don't like the experience.
I have enjoyed all my cruises so far but my last one on NCL was incredible! I really like their product and the Free Style cruising is great, so when answering a experienced cruiser I tell them I like NCL the best.