Conde Nast do a yearly survey - generally Seabourn, SilverSeas and Crystal come out on top. Celebrity wins the best 'large' cruise ship category.
That really depends on your budget!! Otherwise impossible to answer.
I think the best cruise ship is the one on which you prefer travelling. The Berlitz guide awards the highest marks to the Hapag-Lloyd "Europa".
For me the Navigator of the seas and the Independence of the sea.