Oh my -- that's a tough one as they have different "personalities."
Of course, my favorite was SS Norway due to her history, layout, and decor. We've also been on Dreamward, Windward (both pre-stretch), Majesty (before and after stretch), Seaward (now Sea), and Leeward. Overall, we loved the Leeward, which is no longer with NCL. 
. Good cabins and great size, even though the stern was a bit odd.
I love Norwegian Majesty. The sitting area on Deck 5 is my favorite place. But the Dreamward and Windward were much better for sunning, with the tiered decks on top. And I love the open, sloping decks at the stern. I hear the flow on these ships have changed considerably since the stretching.
Norwegian Sea was comfy -- small cabins, small public rooms, yet lots of passengers. Still, she had a charm to her.
I guess I'll have to try them again to make a full assessment.
