I love the Olympic class ships, but I would probably have to choose Lusitania. What a magnificent vessel. My favorite feature of the Lusitania is her opulant two-deck first class dining room. I'd give anything to go on those old liners.
As far as modern liners go, I would have to choose anything by Cunard. I think they build the nicest modern liners. Cruise ships have to be some of the ugliest ships around - both inside and out. Ocean liners like Cunard's ships have better lines.