The jetliners/spaceships of their day. I say that because they were the fastest mode of intercontinental travel, ala jetliners. They were also often sources of national pride, with the best architects, engineers, shipyards using the best materials to make the biggest, fastest, and grandest liners to flex in other nation's harbors.
Of course, the market dropped out when airplanes could ferry people across the Atlantic in hours, not days. Modern cruise ships, bloated floating resorts, are not even the same species as cruise liners