Any Black unit is well worth it. From currently existing this side of the net, top of the line is (fanfare) Nanaly.
Most sought after, once again this side of the globe? Yurina Oh, and Kagerou. And, from now on and till CC rebalance patch, Cecily
In term of scenes? Hmm, maybe Mischa or Nenya or... ...What did you expect? It is entirely a matter of preferences.