You'll get as many answers to this question as there are people; everybody likes something. And what one person likes, another will hate. You'll find somebody that absolutely loves every cruise line out there. This question has been asked a couple of times on here and you'll find everyone has their favorite - every cruise line is someone's favorite.
A favorite cruise line really depends on one's likes and dislikes. This can be based on such things as budget, past experience, age, whether you're single, a couple, or a family, and what your expectations and requirements are for a cruise.
With that said, personally we like Celebrity as our favorite. Followed by Holland America and then Princess. We're an older couple and these cruise lines suit our needs better than others.
But at the same time, we've traveled on all the major cruise lines and have never really had a bad cruise. So, after 41 cruises, we have thoroughly loved every cruise, every ship, and every cruise line we've been on.