Well Shirley shot someone for lelouch, but was easily manipulated by mao to shoot lelouch. and I don't think she can handle him as zero.
and kallen ran away after she found lelouch was zero. instead of shooting spinzaku.
so that would make it seem like C.C or Kaguya would seem better. but kaguya can't pilot a knightmare so that would seem to bring it down to C.C. for usefulness