To tell you the truth, I think I don't really have a "favourite female character" from HP, not even Hermione, who is so brainy (which is something I love in both male and female characters) because she's very annoying sometimes. But Bellatrix is evil and crazy and Helena Bonham-Carter did an amazing job playing her role in the harry potter films and that was crucial to my answer. Of course, I also love Molly Weasley and Tonks and Lily Potter... There are a lot a good female characters in this fandom and I think it's hard to choose.
But what can I say? I love evil characters!