I exclude Marie Rose from my "analysis" because in almost all these packs she has always got outfits enough different from being considered original, and very likey my choice on a favourite one would end to be on one of the outfits worn by her (and so not on the pack in itself).
I believe that my favourite one between the DLC packs listed is the Police one, followed from the pajamas section of the Sleepwear (if you meant it including even the "Bathtime" outfits).
Also, I think that the Schoolgirl pack should be added to the pool because, even if made of different outfits, are anyway a fetish category in itself.