Hachikuji Mayoi! Such a nostalgic feeling character, she is probably my favorite heroine of the Monogatari series for the lively banter in her scenes and the sense of humanity in her personality. From absurd moments to guiding the immature main character, it's never a dull moment! Being one of the sanest AND cutest girls in the show certainly helps too! Karen would be tied or a close second, but it's really tough to choose when they all have a lot of merits.