Each relationship was entertaining and interesting in its time, but Angel/Nina has always intrigued me, simply because there was never time to develop its obvious potential. Angel cared enough about Nina to try and make sure that she left LA before his final battle and to promise to meet her there if he survived, a commitment that he had never made before, even with Buffy. And, according to IDW's "After The Fall" arc, which is included in canon by Joss, Nina, knowing that something very bad was about to happen, chose to stay. There just HAS to be a story there. Unfortunately, beyond that single arc, their relationship was never explored, and probably never will now.