Eirin's midboss noncard is enough bullshit to win the vote for survival alone, even while it is fully static. So my first vote probably goes for midboss Eirin, 6a, since, while
you can cheese Earth in a Pot, and learn the noncard, you still mess it up every now and then.
Sakuya follows close after, since Eternal meek can be bullshit when scoring and the noncard is really wall-happy.
Scorewise, I'm not sure. UFO might be hard, but I've never really tried it, Orin just gets bombed and you usually pick 6B if you go for score in IN. Granted, Eternal meek safespot is not funny at all, but I'm unsure if Youmu is harder when scoring or not. At least on the lower difficulties, where I've tried it, the slash-grazing is no mean feat to pull off consistently. But then again, you COULD just sit in the safespots, but that would not be fun at all, would it? Maybe Youmu or Sakuya, if we're talking about scoring. - But since I haven't really tried scoring on lunatic and especially not the cool grazing tricks that late into the game, I might just as well be wrong.