Considering you are in your 20s I would suggest one of the big three; Carnival, Royal Caribbean or NCL.
If you enjoy fun people, the Carnival is probably the best bet. They go out of their wway to create an atmosphere of "fun." The music and entertainment will be more to your liking, as well as the decor, etc.
If you are sports and fitness minded I recommend Royal Caribbean's bigger ships (Voyager, Freedom or Oasis classes) - they have rock climbing, inline skating, ice skating, etc. But you probably find the staff as outgoing or the food as good as Carnival.
NCL is in the middle. They are activities and a wide variety of food. Maybe not the same service as carnival, though. The ships are fun and the netertainment is good. If sailing after July, I would consider the new Norwegian Epic.