Not sure if the word "NTR" is right or not, but I wanna read shoujo or josei which at the end of the story, the lead female/male is on the losing side. She/he didn't ended with anyone and her/his rival won the love. Nice example of this kind manga are Andante, Kanojo no Namida ga Yuki da Toshitara and Moyashi Otoko to Tane Shoujo.