Here are the nominees so far for the scariest girl in anime:
Aizawa (Great Teacher Onizuka)
Asakura (The Melancholy of Haruhi Suzumiya)
Chloe (Noir)
Diva (Blood Plus)
Dokuro (Bludgeoning Angel Dokuro-chan)
Enma Ai (Hell Girl)
Kaede (Shuffle)
Kansaki (Great Teacher Onizuka)
Kirika (Noir)
Lain (Serial Experiments Lain)
Lucy/Nyu (Elfen Lied)
Marie (Umineko)
Miyu (Vampire Princess Miyu)
Nanami (Katanagatari)
Punie (Magical Witch Punie-chan)
Rena (Higurashi)
Rico (Gunslinger Girls)
Road Kamelot (D. Grey Man)
Satomi (Shadow Star Narutaru)
Shion (Higurashi)
Yomi (Ga-Rei Zero)
Who else? Also, would someone care to nominate a specific Claymore?
Post script: It would be helpful if you name specific characters. I haven’t yet seen Black Lagoon, and I will probably never watch School Days, so I don’t know which characters are which in those. Thank you.