I dunno. May, Dawn, Flannery, Whitney, Jasmine, Clair and Sabrina have always been may favourite Pokemon Characters out of the female selection. But I'd have to choose Whitney since she's my favourite gym leader in Johto!
Sabrina, Jasmine, Clair and Flannery are also my other favourites in their respective region, but Whitney takes the cake. May and Dawn (Platinum Version) are my favourite Pokemon female characters that are actually playable as. But Whitney still takes the cake.