Keiichi is the cast's motivational speaker, and the infamous "pervert speech" easily gets him a vote. He's utterly shameless, and that makes him awesome.
Rena is sweet and cute, yet at the same time competent and clever. Her ending moments in Minagoroshi are a testament to her will. Most of the great moments with her are in season 1, but it's good to see her well in Kai.
Mion gets a place for being the only mostly sane character in a story that really needs one.
She's a ton of fun in the club activities, though her most important moment is probably when she shields the club from Takano.
Rika is the star of the show, and she deserves that spot well. The two sides of her make a very interesting and complex character-- one being innocent and cute, the other being serious and jaded. The balance of the two made Kai a very interesting show to watch.
Hanyuu was just a really good foil for Rika. Her interaction with Rika helps drive the story.
And last but not least, Akasaka , for deciding that he now knows kung fu.
