If it help people, reaally depends on how you play this game, i am the following pirority.

Collect as many different knights as i can (note not all the girls, that is not possible with out not just whaling, but like way too much money)
Collect the girls i prefer, for me that the endowed knights.
Collect a balance of the different types
I prefer multi hit vs single hit if i bother looking at skills.

So to answer the topic, i have three others and currently looking at Mei or Cactace, and i got lucky and got Cactace Bride earlier so Mei it will be if i buy, which i probably will, Having spent double for some of the previous one i bought.