All the girls were great. Rikka had the cute (especially when she gets smacked around), Dekomori was adorably crazy and some good emotional moments in the end, Kumin fulfilled her role as the obligatory clueless bystander with occasional snippets of wisdom, and Nibutani was one of the strongest characters in terms of development.
And then there is hot damn diggity dang hot coochie momma super sexy Touka. So she's got my vote.
I was going to include the minor female characters, but I didn't really think many people would choose them seeing as the focus is on the characters I listed.
I think my second favorite would have to be Nibutani, she grew on me throughout the series, although I still think the hair clip looks ridiculous and thrown on. Touka has third place