All the suggestions so far have been great, but to me the greatest ships to add on to the sim are ones with some historical value to them. I would pick the following (not listed in any real order)
Carpathia, Normandie, Queen Elizabeth, Queen Mary, France, Canberra, Queen Elizabeth 2, Andria Doria, Michelangelo, United States, and NS Savannah.
We all know what those ships look like, but for some of the more obscure ships i would love to see :
The Admrial :
The Aquarama :
The Badger :
The Ticonderoga
And one that has a lot of special, personal memories and meaning to me:
The Mount Washington :The origional before she burned (built from the sistership of the Ticondaroga)
And as she looks now :
The hull is actually over 100 years old, with major upgrades and renovations to it.