Carnival - great if you are young or young at heart and love to party. Really good food.
Holland America - great if you are older (for Alaska), beautiful ships, so so food.
Royal Carribean - great for the 40s to whatever crowd. Gorgeous ships, lots of good activities, food is ok.
Big Red Boat is no longer sailing but they had the best food on any cruise line I have been on
American Hawaii also not sailing, but cruising is a great way to see thoses islands, NCL does it, as does Princess, I believe.
Disney Wonder - beatuiful ship, lousy food (does cafeteria/ institutional food mean anything to you?), service was the worst in the DR. Lots of kids, but well supervised and not running all over. Best private island in all of cruise lines. Shows were good, but the one I like the most is no longer (Herdules) I think. I am sure that it was just a bad set of DR attendants, but it really turned me off to
DCL, sorry.
Even with in each line the ships are different, go to a cruise specialist, let them know what you like, and they can match you up with a good fit line, ship and itinerary.