I am partial to the older ships; beauty and grace of lines were part of the package then. Sagafjord and Vistafjord, now the Saga sisters, are probably the best. Another one is Song of Norway, now Dream Princess. This is somewhat unique, in that she has become even better looking with age. She came into her beauty only after RCL's lengthening improved her proportions. Now as Dream Princess she looks, I think, her best ever. Losing the lounge on the stack helped her lines, and now the dark hull adds to her appearance.
With a few exception mentioned by others already, too many of the newbuilds look like boxes with a pointy end welded on; more like car carriers with lots of windows and balconies. RCCL's latest, the 158,000 ton Freedom class, look boxy because of so many decks crammed on, but I am impressed by the sheer line. It really is nice, and with 2 fewer decks, those ships would be truly beautiful.