God, I have SO many...
But right now, it's that lovable psychotic love starved virgin Hitagi Senjougahara [http://shirouspade.files.wordpress.com/2010/03/senjougahara_hitagi_by_atom_ix2.jpg]. Seriously, mess with her and she'll mess you up. With school stationery. :S
Honorable mentions go to: Death the Kid [http://images.wikia.com/souleater/images/a/a1/Death2.jpg], Osaka [http://3.bp.blogspot.com/_kvhrYHvQTuY/S-FwzMqbENI/AAAAAAAAAE0/dWP2c5bW9-E/s1600/Azumanga+Daioh+Osaka+Knife.jpg], Mitoko "Railgun" Misaka [http://1.bp.blogspot.com/_1u2OzX9_1-0/TCdV__3cXrI/AAAAAAAAACM/E5nxCSkuQoc/s1600/Konachan_com_-_65450_misaka_mikoto_to_aru_kagaku_no_railgun.jpg], Suigintou [http://www.otaku-complex.com/wp-content/uploads/2009/04/suigintou-4.jpg] and Mio [http://www.moeside.net/weblog/wp-content/uploads/2009/KONMioMadness_124B6/moe70164akiyama_miokon__thumb.jpg].