Already have Black Baccara and Alpinia, so it would be down to Sakura, Mei, or Cactacae.
2nd abilities are a bit meh for all, although I think Cactacae would be the most useful.
Speed is a real downside for Sakura - my squads are all between 560 and 660; she is so slow that I
would pretty much have to match her with Alpinia to keep the speeds anywhere near balanced.
Mei wins from the point of view of rainbows in class, as I have no pierce rainbows.Hmmm.