I grew up on the 90's Superman and Batman cartoons my favorite female character was Mercy. Harley for me was more comical relief but I always found Mercy more interesting because she while loyal to Lex was smart enough to realize he was selfish and narcissistic my favorite moment was when Lex called her in that Justice League episode and she just hung up on him.
Mercy is smarter and easily more complex than Harley which is why I don't understand why Harley gets so much attention.