Depends entirely what you want and if in doubt get a copy of the Berlitz guide to cruising, which explains the differences in great detail. In summary mass market and a bit holiday campish but good for the family = RCCL and P&0: steady, no frills very middle class and 50+ = Fred Olsen : more formal, better food and more stylish but more expensive = Cunard, Celebrity:
cheap and cheerful = Thompson. Without doubt the clasiest is Cunard but you will find that most of the better cabins on the Baltic cruises of Queen Victoria for 2008 have probably already gone.
It depends what you want from your cruise. P and O is certainly not holiday campish, its fine cuisine, fantastic entertainment, wonderful staff, formal and informal dining. RCCL is more relaxed, less formal. Cunard and Celebrity give you that little bit extra more than P and O. Celebrity shore excursions are very expensive. Thomas Cook is like butlins, reflected in the prices. My personal opinion. Fred Olsen ships are smaller, and tend to cater for the older clientel.
Having done the Baltics this year with Fred Olsen cannot speak more highly of them and knowledge of local areas that they visit in the Baltics.Staff everywhere on board friendly, curtious and very helpful at all times. We are in our 40's and took our 16 year old daughter who wants to continue cruising she enjoyed all the activities and atmosphere so much .
Did one this year with fred olsen on black watch,excellent highly recommene.