I want to say Pinkladies for 3 reasons:
- She is slightly broken and can save you in many battles.
- Her look and her skill fit perfectly for a rainbow.
- She is my second promoted girl and I don't regret it at all.
Cypress Vine is really good but only for her stats and abilities. She don't looks like a 6* at all and she cost more Crystals than Pinkladies.
Black Locust is good too but there is so much Crit girls in the game... you never know when a better one will comes.
Promotion is a really serious thing. I think you must promote only girls who reach the perfect balance of Waifu and Power.